External drives · Mounting and drive problems
Why won't Disk Utility let me erase my external drive?
Short answer: Select the top-level device (View → Show All Devices), not the volume, and quit anything using the drive — including Time Machine — before clicking Erase. A write-protect switch or a failing drive can also block it.
Most erase failures come from erasing the wrong level or from something still using the drive. Choose View → Show All Devices in Disk Utility, select the top-level device (the drive's model name, not the volume under it), quit apps that have files open on the drive, then click Erase.
Other causes:
- "Couldn't unmount disk" — an app, Spotlight or Time Machine is using the drive. Quit apps, pause Time Machine if it backs up to that drive, and try again.
- A write-protect switch on an SD card or some USB sticks.
- A failing drive that can't be written to.
Erasing removes everything on the drive. If Storage Studio moved items to it, bring them back first or use Transfer Drive to move them to a new drive — see how to remove Storage Studio's folder from a drive.