Choose RAID by balancing usable capacity, tolerated drive failures, workload performance and what your NAS or controller actually supports. For equal-size drives of capacity S, common estimates are RAID 0 = N×S, RAID 1 = S (two-drive mirror), RAID 5 = (N−1)×S, RAID 6 = (N−2)×S, and RAID 10 = roughly half of N×S. These are planning estimates, not promises of formatted free space; use the Western Digital RAID Capacity Estimate Calculator and then verify your system’s documentation.
How much usable space will RAID give me?
Start with the number of drives (N), the usable rated capacity of each equal-size drive (S), and the RAID level. The redundancy rule determines how much raw capacity is consumed by mirroring or parity.
| RAID level | Estimate with equal-size drives | Minimum drives | Drive-failure tolerance |
|---|---|---|---|
| RAID 0 | N×S | 2 | None |
| RAID 1 | S for a two-drive mirror | 2 | 1 drive |
| RAID 5 | (N−1)×S | 3 | 1 drive per set |
| RAID 6 | (N−2)×S | 4 | 2 drives per set |
| RAID 10 | About 50% of total raw capacity | 4 | 1 drive in each mirrored pair; two failures in one pair can defeat the array |
Western Digital summarizes the choice this way: “It depends on how you want to balance protection, capacity and speed.” Its calculator applies the drive count, drive size and selected level to produce an estimate; the final operating-system and filesystem capacity can be lower. Dell’s RAID-selection guidance describes the same trade-off in a specific PowerVault implementation.
Worked example: eight 4 TB drives
Western Digital’s calculator gives these raw usable estimates for 8 × 4 TB:
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#1 Best Overall
- 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.
| Level | Estimated capacity |
|---|---|
| RAID 0 | 32 TB |
| RAID 5 | 28 TB |
| RAID 6 | 24 TB |
| RAID 10 | 16 TB |
These figures are vendor examples, not formatted-space guarantees. A system may report less because drive manufacturers use decimal units while operating systems often display binary units; Western Digital illustrates that a marketed 4 TB drive can appear as approximately 3.64 TB before filesystem overhead. Treat that conversion as the vendor’s explanation rather than an exact result for every platform.
Which RAID level should you choose?
There is no universal best level. Match the level to the consequence of a failure, the amount of capacity you can sacrifice, the I/O pattern and the capabilities of the target hardware.
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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
RAID 0: maximum capacity and striping, no protection
RAID 0 stripes data across at least two drives and makes all raw capacity available. Any drive failure can make the array unavailable, so it is appropriate only when the data can be recreated and performance or temporary workspace matters more than availability.
RAID 1: simple two-drive mirror
RAID 1 stores duplicate data on two drives, yielding one drive’s capacity and tolerating one failed drive. It is easy to understand and commonly suited to small systems where recovery simplicity matters more than capacity efficiency.
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- 【Reliable External Storage System for Individuals and business】The 3.5 hard drive enclosure supports 2.5/3.5 inches HDD and SSD, max capacity up to 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.
- 【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.
RAID 5: one-drive parity protection
RAID 5 needs at least three drives, reserves the equivalent of one drive for distributed parity and tolerates one failed drive per set. Dell describes RAID 5 as offering good sequential and moderate random I/O performance in its ME4 guidance. Rebuild time and risk depend on the controller, drive size, workload and array condition, so do not treat that description as a universal benchmark.
RAID 6: two-drive parity protection
RAID 6 needs at least four drives, reserves the equivalent of two drives for parity and tolerates two failed drives per set. Dell characterizes RAID 6 as having moderate sequential and poor random I/O performance on the ME4 platform, while Seagate notes that its additional parity calculations can make rebuilds slower than RAID 5. Those are implementation-specific observations, not guarantees for every NAS or controller.
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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.
RAID 10: mirrored pairs with striping
RAID 10 combines mirrored pairs and striping, requires at least four drives and provides about half the raw capacity. It can survive one failed drive in each mirror pair. If two drives fail in the same pair, that pair has no remaining copy and the array can fail; the exact outcome therefore depends on which drives fail, not merely the number of failures. Dell describes RAID 1/10 as providing good random-I/O performance with a 50% capacity cost in its ME4 documentation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How many drives do RAID 5, RAID 6 and RAID 10 need?
- RAID 0: minimum two drives; no fault tolerance.
- RAID 1: minimum two drives; one-drive tolerance for a two-way mirror.
- RAID 5: minimum three drives; one-drive tolerance.
- RAID 6: minimum four drives; two-drive tolerance.
- RAID 10: minimum four drives, normally an even count arranged as mirrored pairs; one failure is tolerated in each pair.
These minimums come from Western Digital’s calculator guidance. A particular NAS, hardware RAID controller or software implementation may impose different limits, so confirm the exact model’s supported levels and drive counts before buying disks.
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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
What happens with different-size drives?
In standard RAID, capacity is commonly based on the smallest drive. Western Digital’s calculator illustrates two 4 TB and two 12 TB drives in RAID 5 behaving like four 4 TB drives, leaving 16 TB of the larger drives’ nominal capacity unused. This is an illustrative calculator example, not a rule for every vendor’s flexible or hybrid RAID technology. Check the implementation’s documentation before mixing sizes.
How to use a RAID calculator without overestimating capacity
- Count the drives that will belong to the array. Exclude a separate hot spare unless the calculator or controller explicitly treats it as part of usable capacity.
- Record each drive’s rated capacity and interface. If sizes differ, use the smallest size for a conventional-array estimate unless your system documents another method.
- Select only a RAID level supported by the target device. Intel’s support article lists RAID 0, 1, 5, 6, 10, 50 and 60 for the controllers it covers, but that list does not mean every motherboard, NAS or controller supports all of them. See Intel’s supported-levels documentation.
- Enter the values in the calculator. The Western Digital calculator reports a level-based estimate.
- Discount the result for real-world formatting. Operating-system unit conventions, partitioning and filesystem metadata reduce the number shown as free space.
- Leave operational headroom. Do not plan to fill an array to the last displayed gigabyte; snapshots, metadata, alerts and rebuild activity need room according to your platform’s guidance.
Capacity is only one part of the decision
Compare each candidate on four separate questions:
- Failure exposure: How many failures can the level withstand, and can particular combinations (such as both members of one RAID 10 pair) defeat it?
- Workload: Random I/O, sequential transfers, latency and write behavior can favor different levels. Vendor descriptions are tied to specific controllers and should not be converted into a universal speed ranking.
- Rebuild implications: Parity calculations and the amount of data to reconstruct affect rebuild duration and performance impact. A larger array may offer more capacity while taking longer to restore redundancy after a failure.
- System support: The NAS, operating system, HBA or hardware controller must explicitly support the level, drive count, maximum disk size and expansion behavior you need.
Choosing drives and checking compatibility
For a system intended to run continuously, shop in the category of NAS hard drives rated for the planned RAID environment. Western Digital identifies its WD Red Plus and Red Pro lines as NAS-oriented continuous-operation products, but no generic guide can promise that a particular model works in every enclosure or controller.
- Open the NAS or controller’s current compatibility list before purchase.
- Check supported interface, sector format, capacity limits, firmware requirements and approved drive families.
- Confirm that all bays can use the intended size and that the enclosure supports your chosen RAID level.
- Plan replacement availability and a tested procedure for replacing and rebuilding a failed disk.
RAID is not a complete backup plan
RAID is designed to keep an array operating through specified drive failures. It does not by itself create an independent copy of data or establish recovery from events outside that failure model. Keep separate protection for the data your organization cannot recreate, and verify that you can actually restore it.
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