Assuming Vista or 7 (this should work on XP, 8, 8.1, and 10, as well) and that the disk is not showing up under My Computer at all:
- Connect your disk.
- Run
cmd as an Administrator.
- Run
diskpart.exe. Use ? if you need help in this program.
list disk- Find the disk that corresponds to your USB disk.
select disk n where n is the number of the disk. Confirm that you're using the right disk with detail disk.
clean (Warning: This command erases the disk's partition information. Any data on the disk will no longer be accessible.)create partition primary. No size is needed if you want to use the whole diskactive. Optional. Marks the partition as potentially bootable.format fs=fat32 quick. You can choose NTFS or exFAT instead of FAT32 if you want. (Note: Windows 10 limits FAT32 to 4GB. I recommend using exFAT instead, which is essentially a newer version of the same format.)assign. Assigns the disk a drive letter.exit to quit.
If you're still having problems with the disk after trying this, you might try omitting the quick from step 9 to do a full format. That will take a very long time and usually isn't necessary, but may help uncover physical damage to the disk.
In some cases you might need to re-initialize the disk. As above, this will destroy the data on the disk (or, rather, your ability to access it). To do that from diskpart.exe:
list diskselect disk nattributes disk clear readonly. This command will unset the read-only flag on the disk. You can see the current attributes with attributes disk or detail disk.online disk. Sets the device status to online if it's been disabled.convert mbr. Converts the disk to MBR format, which will work just fine for most disks. If you've got a USB disk larger than 2 terabytes, however, you'll want to use convert gpt instead.
I'm not sure if the above commands are all identical on older OSs (XP).
