I have a 32 GB USB flash drive which is currently formatted to 261 MB:

Screenshot of a dialog in the disk utility

How can I reformat/erase that USB drive so that I access all 30.75 GB of it?

Disk utility won't let me change the size of the partition to 30.75 GB.

Update 1: I erased the drive. At the end, there was this dialog box.

Erase dialog screenshot

Update 2: I took the path of least resistance and changed the size of the drive to 30 GB on a Windows machine (discmgmt.dsc utility).

  • Are you sure it is 30G - there are a lot of USB sticks that were fraudently sold as large but were only a very small size
    – mmmmmm
    Commented Jan 16, 2021 at 13:08
  • @mmmmmm I am not 100 % sure, but I do think it is much larger than 260 MB for two reasons. First, it's a SanDisk (a reputable brand). Second, after I bought it, it had several gigabytes of space on it. Then I copied the ISO image of a utility onto it and made it bootable. That utility took a couple of hundreds of megabytes. I assume that the amount of space on this drive shrunk as a result of putting that iSO image on it. Commented Jan 16, 2021 at 14:50
  • Can you change it to 29 GB?
    – Natsfan
    Commented Jan 16, 2021 at 15:35
  • 1
    Have you tried erasing it in disk utility?
    – X_841
    Commented Jan 16, 2021 at 15:38
  • 1
    Just to be extra sure: You open disk utility, you have selected show all devices, then select the top most entry referring to your usb stick and try to erase it? And after erasing you still only have 260mb?
    – X_841
    Commented Jan 16, 2021 at 21:06

2 Answers 2


I don’t know a good way to test disks for sizing without a third party program or command line tools.

The former, I use SD Clone: https://twocanoes.com/knowledge-base/sd-clone-3-getting-started/

The latter - try these commands once you get the unix disk number from Disk Utility or another means - be 100% sure you have a backup of every single drive connected. If you get the drive number wrong (in the linked question the USB drive to be erased is disk3 and those numbers can change each time you eject and mount a drive), you will erase without a chance of recovery whichever drive you perform this on:

diskutil list diskX
diskutil eraseDisk JHFS+ TestDisk disk3
diskutil list diskX

When I say “be sure of your backup”, that you have set aside the time and steps needed to restore that backup - not just “maybe it’s current and I haven’t checked it for a while” type backup. Hopefully it’s a quick erase and not that the disk itself is broken or mislabeled.


If you can you might trying erasing the disk and then reformat it. You will, of course, lose everything written to disk. If you want to keep what's on the disk you could copy it you main drive. Disk utility allows you to erase the disk and then reformat it.

After opening Disk Utility go to the Edit menu and at the top of the panal you'll see option to erase, partition, and other commands.

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