At System Preferences > Startup Disk I select a disk with Mac OS X (10.5 or 10.6) installed, but when I click Restart I get this error message: You can’t change the startup disk to the selected disk. The bless tool was unable to set the current boot disk.

I have 10.6 installed on an internal hard drive and 10.5 on an external drive.

I have completely erased both disks from Disk Utility; done Verify Disk, Repair Disk, Verify Disk Permissions, Repair Disk Permissions; reinstalled Mac OS several times on both drives...


Looks like something is wrong with internal hdd. I have tried to restart the machine in target disk mode, but I got You can't start up with target disk mode. The nvram tool was unable to set a preference

  • Can you select and boot to the 10.5 (external) volume from the EFI boot manager (get to this by holding the ALT key before the chime and apple logo at boot)?
    – macaco
    Commented Aug 5, 2011 at 15:43
  • How did you install OS X onto the drives?
    – dan8394
    Commented Aug 5, 2011 at 15:44
  • Using Refit as a boot loader might help you. refit.sourceforge.net
    – dan8394
    Commented Aug 5, 2011 at 15:45
  • @macaco: did not try, will try and let you know Commented Aug 5, 2011 at 20:04
  • @dan8394: the usual way, inserted OS DVD, selected drive where to install OS... Commented Aug 5, 2011 at 20:05

2 Answers 2


From the errors given, it sounds like it's more about setting the NVRAM than the startup disk itself. Clear the NVRAM (previously referred to as PRAM; reboot holding command-option-P-R until you hear a second chime) and see if the problem works itself out.

  • Are you sure your volume has a valid bootable image? That will cause bless to fail - you may have to run the bless command in the terminal or look at the logs to discover this error.

  • What is the partition map on the drives set to? Disk Utility will give you three options: Apple Partition Map, GUID, and Master Boot Record. For any relatively recent Mac, you should always choose GUID. For PowerPC Macs, choose APM. If your disk is MBR, you will not be able to boot OS X from it.

When in doubt, these days, choose GUID. To change the partition map, you'll have to reformat the whole disk (including wiping all partitions).

  • No mac will bless a volume contained in a MBR. It is very lenient in blessing APM and GUID even if the hardware doing the blessing won't boot a USB drive. You could be preparing it for a mac that does boot in the combination you ask.
    – bmike
    Commented Aug 5, 2011 at 16:03
  • @bmike That's what I thought, but I wasn't entirely sure if MBR was verboten. It appears to be; I'll edit.
    – Cajunluke
    Commented Aug 5, 2011 at 19:19
  • Yes, I was able to boot from both drives. I will check partition map, but I am pretty sure it is GUID. Commented Aug 5, 2011 at 20:08
  • Looking at the external (lacie) hdd, Disk Utility says Partition Map Scheme: GUID Partition Table. Commented Aug 8, 2011 at 11:56
  • Internal (hitachi) hdd the same: Partition Map Scheme: GUID Partition Table. Commented Aug 8, 2011 at 11:57

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