If a USB flash drive or SD card is missing from File Explorer or Finder, first check whether the computer detects it in its disk utility. A missing drive letter or an unmounted volume can make storage invisible without proving the device has failed. If the device contains files you need, do not initialize, format, erase, or delete its partitions while troubleshooting: those actions can destroy data.
Start with safe physical checks
- Make sure the flash drive or card is fully inserted and correctly oriented. Check the connector and adapter for visible damage.
- Try a different USB port. For an SD or microSD card, use a reader that supports that card type and format.
- If possible, test the card or drive in another compatible reader or computer. If it works there, the first computer, port, built-in reader, adapter, or configuration becomes a more likely cause, though that comparison cannot identify the exact fault. SanDisk recommends checking removable storage on another computer in its Windows and macOS troubleshooting guidance and USB and card-reader tips.
Next, inspect the system disk utility. A device listed there is detected at least in part even if it does not appear in the file browser; this does not establish that its files or file system are healthy.
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Check the drive in Windows
Open Disk Management
In Windows 10 or 11, right-click Start and choose Disk Management, search for “Create and format hard disk partitions,” or press Win+R, enter diskmgmt.msc, and press Enter. Disk Management is built into both versions of Windows. Identify the removable disk by its capacity and status before changing anything. See Microsoft’s Disk Management in Windows guidance.
Interpret what it shows
- Healthy volume, no drive letter: Windows may detect the volume without showing it in File Explorer. Assigning a drive letter may restore access. Confirm you have selected the correct removable volume before making the change.
- Disk marked Offline: Disk Management may offer an option to bring it Online. Do not proceed if the status or disk identity is unclear.
- RAW, unallocated, or a prompt to initialize or format: Stop if the media contains files you want. A RAW or unallocated state is not a reason to format or initialize in the hope of recovering access. Microsoft warns that formatting destroys data and advises formatting a RAW volume only when the disk is known to be empty. Its Disk Management troubleshooting also cautions against formatting when recovering files.
- Device absent: Recheck the port, reader and compatibility, then test another computer if available. If it remains absent, the available checks cannot identify whether the drive, reader, connector, or computer has failed.
Some USB disks arrive formatted and assigned a letter, so they do not need initialization. Do not initialize, delete partitions, or format removable media that may contain needed files.
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Check the card or drive on a Mac
Inspect Disk Utility
Open Disk Utility and look for both the device and its volume. If the volume appears but is not mounted, the issue may be its mount state rather than a complete failure. Apple explains how to inspect and repair storage in Repair a storage device in Disk Utility on Mac.
Use First Aid cautiously
Disk Utility’s First Aid can check and fix some formatting and directory-structure errors, but Apple says it cannot detect or repair every disk problem. Apple recommends running First Aid on each volume and container, then on the storage device itself. If First Aid reports that the disk is about to fail, Apple advises backing up the data and replacing the disk. If files are inaccessible and important, prioritize preserving them and consider professional recovery advice before erase or reformat steps; replacing the device is not the same as recovering its contents.
Rank #2
- High-speed USB 3.0 performance of up to 150MB/s(1) [(1) Write to drive up to 15x faster than standard USB 2.0 drives (4MB/s); varies by drive capacity. Up to 150MB/s read speed. USB 3.0 port required. Based on internal testing; performance may be lower depending on host device, usage conditions, and other factors; 1MB=1,000,000 bytes]
- Transfer a full-length movie in less than 30 seconds(2) [(2) Based on 1.2GB MPEG-4 video transfer with USB 3.0 host device. Results may vary based on host device, file attributes and other factors]
- Transfer to drive up to 15 times faster than standard USB 2.0 drives(1)
- Sleek, durable metal casing
- Easy-to-use password protection for your private files(3) [(3)Password protection uses 128-bit AES encryption and is supported by Windows 7, Windows 8, Windows 10, and Mac OS X v10.9 plus; Software download required for Mac, visit the SanDisk SecureAccess support page]
Choose the next step from the result
| What you see | What it suggests | Safer next step |
|---|---|---|
| It appears in Disk Management or Disk Utility, but not the file browser | The computer detects at least part of the device; the volume may lack a drive letter or be unmounted. | Check the volume state and access settings. Avoid formatting if files matter. |
| It works with another compatible reader or computer | The original port, reader, adapter, or computer configuration is a more likely cause; this is not a definitive diagnosis. | Check the original reader and port, and confirm SD/microSD compatibility. |
| Windows reports RAW or unallocated, or asks to initialize/format | The volume is not available in a normal usable state; formatting destroys data. | Stop if files matter. Do not initialize, format, or delete partitions. |
| It is missing from the disk utility on multiple computers | Reader compatibility or hardware failure is possible, but these checks do not identify the failed component. | Check connector and reader compatibility. Avoid repeated repair attempts if the data matters. |
| Mac First Aid says the disk is about to fail | Apple identifies a failure warning, not a guarantee that software repair will restore the disk. | Back up accessible data and replace the disk; seek recovery advice separately if files cannot be accessed. |
When a different reader or replacement makes sense
If a card works in another compatible reader, replacing or troubleshooting the original reader may be more relevant than replacing the card. Check that a replacement reader supports the card format and has the connector your computer uses. If checks point to failed storage, replacement may restore future use but will not recover files from the old drive or card. Handle data recovery as a separate priority before discarding or reusing the media.
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Best Value
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Rank #4
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Rank #3
- What You Get - 2 pack 64GB genuine USB 2.0 flash drives, 12-month warranty and lifetime friendly customer service
- Great for All Ages and Purposes – the thumb drives are suitable for storing digital data for school, business or daily usage. Apply to data storage of music, photos, movies and other files
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