How to Recover Lost Disk Space on a USB Drive
If you’ve ever copied a disk image to a USB thumb drive, you may have have run into the mysterious problem of disappearing disk space. This is normal after you’ve copied an image to the drive. However, if you one day want to reclaim that drive for some other use, it may look as though that extra space is just gone. Don’t worry. It’s not.
When the partition scheme is changed, the size of the drive may appear to have changed. If this is the case, even formatting the drive won’t get the missing space back. I have heard of this happening on Windows more than anything else. I have included instructions here for both Windows and Mac OS X.
In order to correct this problem, we need to repartition the drive, then reformat it. That’ll restore the drive to it’s original glory.
Before proceeding, please be sure to back up all of your files. While I don’t expect anything to go wrong, it’s always better safe than sorry.
If you’re on a Windows machine, keep reading. Otherwise, skip to the next section for Mac OS X.
Open the DiskPart utility. Likely it came packaged with Windows. You can get to it by going to Start, then typing diskpart
When DiskPart starts -- in a command line window -- type list disk and hit enter
There will be several disks listed with labels like Disk 0, Disk 1, and their sizes. Choose the disk that’s the same size as the USB drive and make absolutely sure it is the correct one. This procedure is going to repartition the disk and erase everything on it so make sure the correct disk is selected! As usual, keep backups.Once you are certain which disk is the correct one from the list, type select disk # (replace the # with the actual disk number).
Now type clean to reset the partition.
Once that command completes, you should see a message saying something to the effect of DiskPart succeeded in cleaning the disk.
Mac OS X
Partitioning the disk will erase everything on it. Make sure there isn't anything on the disk that shouldn't be erased and that the correct disk is selected from the list.
On OS X we're going to use Disk Utility. You can find this handy app in the /Applications/Utilities/ directory. It is also easily accessed by opening Spotlight (press command + spacebar) and typing Disk Utility. Actually, just typing in disk u will probably bring it up.
Next, click on the disk you want to repartition. Click the Partition tab, then select "1 Partition". Make sure the size of the partition is about the same size as the actual disk. Now just hit Apply and you're done!