RAID 1 is disk mirroring: every block written to one drive is duplicated on another mirror member. In the common two-drive setup, the operating system gets usable capacity equal to one drive, while the second drive provides continuity if the first fails. That protection is limited to a drive-failure scenario; RAID 1 is not a backup.
What is RAID 1?
RAID 1 is a redundant array layout that maintains identical data on two drives. Seagate’s RAID terminology describes writes being sent to both members and reads being serviced from either member. Intel describes a two-drive RAID 1 volume as mirrored in real time.
The mirror can be implemented by a hardware RAID controller, a NAS appliance, or operating-system storage software. The exact management screens and recovery procedure depend on that platform, so use the controller or NAS manufacturer’s documented process when creating, replacing, or rebuilding an array.
How does RAID 1 mirroring work?
Writes are duplicated
When the system writes a file, the RAID implementation records the data on both mirror members. A successful write therefore leaves two copies on separate drives rather than one combined block of capacity.
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- Note:The eSATA port on this product does not support the use of a computer’s SATA-to-eSATA adapter. Hot-swapping is not supported. The computer’s eSATA port must support RAID functionality to properly access multiple drive bays via the eSATA port; otherwise, only one drive bay can be accessed.
- 【Reliable External Storage System for Individuals】The 3.5 hard drive enclosure supports 2.5/3.5 inches HDD and SSD , max capacity up to 80TB( 20TB for each hard drive), it's a ideal external hard drive enclosure for personal or enterprise using.Save space on your desktop or laptop.
- 【No heat,】The 4 bay hard drive reader built in Aluminum-Alloy materials and 2 inch Fans.Maximize the security of your data.NOTE：Fan noise is around 40-50 decibels, not recommended if you are very sensitive to noise.
- 【8 Raid Modes】This external hdd raid enclosure supports RAID 0/1/3/5/10, CLONE, LARGE, NORMAL.NOTE:When replacing RAID, you need to go back to NORMAL and set the desired RAID mode.Designing RAID may result in data loss.MAC OS no Raid software. Raid Mode Switching Method Disconnect the power, use a screwdriver, toggle the paddle to the corresponding mode, press and hold the reset button, turn on the power, hold reset for ten seconds, the raid mode will be successfully switched.
- 【Up to 5Gbps】This raid enclosure equips with JMS567+JMB393 chip and USB 3.0, eSATA output interface.
Reads can use either member
Because each member contains the same data, a controller can read from either drive. Some implementations can distribute read requests, but read-speed behavior varies with the controller, software, workload, and queue depth. RAID 1 should not be treated as a guaranteed performance upgrade.
The mirror is one logical volume
Applications normally see one volume, not two independent disks. The RAID layer handles keeping members synchronized and reports the array’s health to the host system.
How much space do you get with RAID 1?
A two-drive mirror provides usable capacity equivalent to the smaller member. Intel’s example uses two 120-GB drives and presents 120 GB to the operating system, not 240 GB. QNAP likewise states for the NAS scenario covered in its replacement FAQ that usable capacity is determined by the smallest member.
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- High Speed Data Transmission: The D2-320 hard drive enclosure (a DAS, NOT a NAS) adopts USB 3.2 Gen2 protocol for high-speed data transmission up to 10Gbps. With 2 hard drives in RAID 0, the read/write speed can reach up to 521MB/s (SATA III HDD 8TB x 2). With 2 SSD's in RAID 0, the read speed can reach 1075MB/s (SATA III 1TB SSD x 2)
- Multiple RAID Configurations: The D2-320 is a hardware RAID enclosure and it supports RAID 0, RAID 1, JBOD and SINGLE which can better satisfy various demands of users. In RAID 1, data will be in a mirror backup. When there is a damaged hard drive, you can directly replace the hard drive, and the data will be recovered automatically. This provides an absolute security for the data
- Super-Large Storage Capacity: The D2-320 USB storage enclosure can support up to two 3.5" and 2.5" SATA HDD, as well as 2.5" SATA SSD, with a maximum capacity of 22TB per drive, providing users with up to 44TB (22TB x 2) of storage space
- Intelligent Temperature Control: The D2-320 HDD enclosure has an intelligent temperature-controlled and low-noise fan that automatically adjusts its speed based on the temperature of the hard disk. This feature ensures that the hard disk operates at its best temperature and provides better heat dissipation
- Tool-Free Hard Drive Installation: The D2-320 external hard drive enclosure features a tool-free hard drive tray design that allows for easy installation and removal of hard drives without the need for any tools. Furthermore, the D2-320 incorporates a brand new Push-lock unique design from TerraMaster, which automatically locks the hard drive tray when you insert the hard drive, preventing the hard drive from falling out or disconnecting
Actual displayed capacity can be lower than the advertised decimal capacity because vendors and operating systems report space differently. If the drives are different sizes, the extra space on the larger drive is generally unusable in that mirror. QNAP’s guidance also requires a replacement to provide at least the existing member’s usable capacity; compatibility rules are product-specific.
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| Configuration | Member drives | Usable capacity concept |
|---|---|---|
| Typical RAID 1 | 2 | Approximately one drive’s capacity, limited by the smaller member |
| Intel example | 2 × 120 GB | 120 GB presented to the operating system |
What happens if one RAID 1 drive fails?
Continued access in a degraded state
Intel documents that a two-drive RAID 1 system can continue operating after one drive fails because the surviving member still contains the data. The array is then degraded: it has lost redundancy and no longer has the same protection against another member failure.
Replace the failed member
Use the platform’s health panel and replacement instructions to identify the failed device, obtain a supported drive, and start a rebuild. Do not unplug or swap a disk based only on its physical position; hot-swap bays and device names differ by system.
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- 【4 Raid Modes】!!!NOTE:Press and hold the "Reset" button for 5 seconds after reset the RAID array!!!This raid enclosure supports 4 RAID Modes(RAID 0, RAID 1, Normal, JBOD).Designing RAID may result in data loss.MAC OS no Raid software.
- 【No heat】The 2 bay hard drive reader built in Aluminum-Alloy materials and 2 inch Fan.Maximize the security of your data.NOTE：Fan noise is around 40-50 decibels, not recommended if you are very sensitive to noise.
- 【Up to 5Gbps】This dual bay raid enclosure equips with JMS561 chip and USB 3.0 output interface.
- 【Wide Compatibility, Plug and Play】Equipped with USB A/C 3.0 Cable.Compatible with Windows 7 and above, Mac 9.1 and above, Linux.Plug and play, no fuss, no muss.
Rebuild restores the mirror
During a rebuild, the system copies the surviving data onto the replacement drive and re-establishes the duplicate. Seagate notes that rebuild activity can affect performance. Until it completes, a second failure can leave the array without a complete surviving copy, so replace a failed member promptly and monitor progress.
Choosing a replacement
QNAP’s FAQ, last modified March 12, 2026, says a replacement in its documented NAS scenario must be supported and have at least the existing member’s usable capacity. It notes that some models may technically support mixing HDDs and SSDs when those conditions are met, while also warning that system design affects performance. Those are QNAP-specific requirements, not a universal rule for every controller.
What RAID 1 protects against—and what it does not
Protection against a single member-drive failure
The primary benefit of RAID 1 mirroring is reliability when one disk fails. The surviving member keeps the volume available while you replace and rebuild the failed member.
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- !!!NOTE:When the 8-bay enclosure being used, there is at least one hard drive must be inserted into HDD1-HDD4, same goes for HDD5-HDD8, 2 HDDs is a minimun quantity to be inserted.Please read the instructions carefully before trying!!!Be sure to save a good backup of your data before setting up RAID, which will format your hard drive after setting up RAID!!!!!!
- NOTE: When using this product, please first confirm that the hard drive loaded into this product is normal, otherwise it will lead to not out of the drive, such as loading more than one hard drive, it will only show one, can not confirm which one is bad, please load a hard drive, power on, out of the drive a, confirm that it is normal, turn off, and then load the second, in the power on, out of the drive two, to confirm that it is normal, and so on, one by one to load, until you find the The problematic hard drive. For example, if there is a problem with one of the 8 hard drives, only one drive will come out.
- 【Reliable External Storage System for Individuals】The 3.5 hard drive enclosure supports 2.5/3.5inches HDD and SSD , max capacity up to 160TB( 20TB for each hard drive), Not compatible with WD 20TB hard drives, but supports Seagate 20TB hard drives.it's a ideal external hard drive enclosure for personal or enterprise using.Save space on your desktop or laptop.
- 【8 Raid Modes】This external raid enclosure supports CLONE, LARGE/ LARGE*2, NORMAL, RAID0*2, RAID5*2, RAID50, RAID00. NOTE:When replacing RAID, you need to go back to NORMAL/PM10 and set the desired RAID mode.Designing RAID may result in data loss. !!!Raid Mode Switching Method!!! Disconnect the power, use a screwdriver, toggle the paddle to the corresponding mode, press and hold the reset button, turn on the power, hold reset for ten seconds, the raid mode will be successfully switched.
- 【No heat】The 8 bay hard drive reader built in Aluminum-Alloy materials and two 2.9 inch Fans.Maximize the security of your data. NOTE：Fan noise is around 40-50 decibels, not recommended if you are very sensitive to noise.
Not protection from data-level mistakes
Both members receive normal writes. Consequently, accidental deletion, file corruption, malware, or an application that overwrites good data is mirrored as well. Fire, theft, controller damage, and loss of the entire NAS or computer can also remove both copies.
RAID is not a backup
Keep an independent backup on separate media or at a separate location. Seagate advises backing up important files before changing an existing array; a backup should remain usable even when the RAID device is unavailable or its data has been changed incorrectly.
Benefits and trade-offs of RAID 1
| Benefit | Trade-off or limit |
|---|---|
| Data remains available after one member fails | Half of a matched two-drive raw capacity is used for the duplicate |
| Simple two-copy model that is easy to monitor | The array is less protected while degraded or rebuilding |
| Reads may be served by either member in some implementations | Read gains are not guaranteed, and writes must reach both members |
| Useful for systems where uptime matters | It does not replace independent backups or protect against logical and site-wide loss |
RAID 1 versus RAID 10
RAID 1 and RAID 10 are related but different layouts. HPE’s Intelligent Provisioning 3.88 User Guide describes two physical drives as RAID 1. With at least four drives arranged as mirrored pairs and striped across those pairs, it uses the names RAID 1+0 or RAID 10.
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- [Flexible RAID Mode Management]: This 3.5-inch RAID HDD enclosure supports eight configuration modes, namely 0, 1, 3, 5, 10, JBOD, CLONE, and CLEAR. It enables dual data backup, enhances data security, and caters to the individualized needs of diverse users. Note: It is advisable to back up your data before mode switching. If you have any inquiries, please do not hesitate to contact us
- [Supports 22TB Single Disk]: The 5-bay HDD enclosure accommodates 3.5-inch SATA disks, and the maximum storage capacity amounts to 110TB. It can effortlessly fulfill the storage requirements of large-scale engineering projects, high-resolution video footages, and other large-capacity data, eliminating concerns about capacity shortages
- [5Gbps Data Transfer]: The USB 3.0 interface of the external hard drive bay is compatible with SATA 6 Gbps, and the transfer speed reaches up to 235MB/s, facilitating effortless backup and transfer of files and videos, enabling centralized management and enhancing work efficiency
- [Effective Heat-dissipation]The 3.5-inch aluminum HDD case is outfitted with an 80mm silent cooling fan. Front and rear vents are designed, and the airflow effectively dissipates heat, ensuring the stable and efficient operation of the equipment over an extended period
- [Safety Protection]: The RAID enclosure features a bracket-free design for quick disassembly and assembly and possesses an independent safety locking mechanism to effectively prevent the unexpected removal or loss of the hard disk and guarantee the security of data
| Characteristic | RAID 1 | RAID 10 (RAID 1+0) |
|---|---|---|
| Layout | A mirrored pair | Striping across multiple mirrored pairs |
| Minimum in the cited configurations | 2 drives | 4 drives |
| Capacity efficiency | About one drive’s capacity from a two-drive pair | About half of the aggregate raw capacity, subject to implementation and drive sizes |
| Failure tolerance | One member failure leaves the surviving member available | Can tolerate multiple failures only when no two failed drives are in the same mirrored pair, according to the cited HPE guide |
Controller terminology varies, so confirm whether a product calls a particular multi-drive arrangement RAID 10, RAID 1+0, or something else. Performance and rebuild behavior also depend on the implementation; avoid universal speed claims.
When is RAID 1 a good fit?
- Small NAS or file server: two matched drives provide continued access if one member fails.
- Workstations with important local data: mirroring can reduce downtime from a disk failure when capacity requirements are modest.
- Boot or application volumes: a mirrored pair can keep a system running while a failed member is replaced, provided the platform supports bootable RAID 1.
- Environments prioritizing simple recovery: the surviving drive contains a complete copy, making the failure model straightforward.
Choose another layout when you need more usable capacity from several drives, a different write-performance profile, or protection against failure patterns that a two-member mirror cannot handle. Compare minimum drive count, usable capacity, tolerated simultaneous failures, workload-specific read/write behavior, and rebuild characteristics for the exact controller or NAS model.
Practical setup and maintenance checklist
- Confirm that the motherboard, RAID controller, NAS, or storage software supports RAID 1 and identify its documented creation procedure.
- Use supported drives with the same or compatible form factor, interface, and capacity rules. For a replacement, follow the platform’s compatibility list rather than assuming any disk will work.
- Back up important files independently before creating, modifying, or rebuilding the array.
- Create the mirror through the platform’s management interface and verify that both members show healthy status.
- Enable health alerts and check them regularly so a failed member is detected quickly.
- When a failure is reported, identify the device using the platform’s serial number or bay indicator, replace it according to the vendor’s instructions, and monitor the rebuild to completion.
- Test restoration from the independent backup; a healthy RAID status alone does not prove that a backup can be recovered.
Common misconceptions
“Two 4-TB drives give me 8 TB.”
No. In a normal two-drive RAID 1 mirror, usable capacity is limited to roughly 4 TB before reporting overhead.
“RAID 1 means the data is backed up.”
No. It is a live duplicate for drive redundancy, not a separate, versioned, or off-device backup.
“A degraded array is safe until convenient.”
No. Degraded status means the remaining member is carrying the only complete copy in the array. Replacing the failed member and rebuilding restores redundancy, and rebuild work can affect performance.
“RAID 1 and RAID 10 are interchangeable names.”
No. RAID 10 combines mirrored pairs with striping and normally requires at least four drives in the cited configuration.
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