I am trying to install Mac OS Big Sur on an external drive (Samsung portable SSD T5 256GB) following the macworld directions. I get to the very last step, however there is a message that the disk is locked and it can't be selected, so can't install macOS on it.

macOS install disk locked

I have previously installed macOS on the drive using restore from backup, but I just want a minimal OS install, not all my stuff from a backup.

I have also tried to select the drive as the boot disk, but get a message that I can't select the drive because "the bless tool was unable to set the current boot disk".

Bless tool unable to set the current boot disk

There is a previous question Disk Locked” macOS reinstall! however I don't think that helps, there's also a post on an Apple discussion forum about the bless tool, but that doesn't help either.

Any ideas on how to rectify this?

1 Answer 1


Of course after hours of trying, I found an answer soon after I posted.

  1. I connected the external drive
  2. From the app store, I found Big Sur and clicked "Get". Because I'd already downloaded the installer, it started after a few moments
  3. I clicked continue and accepted the Ts & Cs
  4. When the "select disk to install" came up, only the internal disk was shown so I clicked the "Show all disks" button
  5. The external disk was also shown, so I selected it
  6. A message said the disk couldn't be used as it wasn't APFS with GUID partition map
  7. I backed out of the installer, ran disk utility, erased the disk and formatted it as APFS with GUID
  8. Ran steps 1 to 4 again, this time the install proceeded
  9. 20 minutes later I had a bootable external drive.

The above is far simpler than the MacWorld instructions.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .