Recently, I purchased an SSD and using dd copied my OSX HDD onto that SSD. I'm able to boot from both drives when they are installed in my Macbook Pro, separately. However, if both drives are in the machine (I swapped out the CD and replaced it with the old HDD), the machine appears to get stuck trying to mount the drives. No boot from either drive is possible with both present.

I'm assuming that dd (which makes an exact copy) has made both drives look exactly the same to the h/w and something is not allowing the bus to distinguish between the two. What (and how) do I change to make it possible to have both drives co-exist simultaneously? I'm thinking it's got to be some identifier in the drive somewhere.

Background: why would I do this and not simply wipe the old HDD? Two reasons: (1) I'd like to have the old drive around as backup for a couple of weeks to be sure the new drive is stable and (2) I've since upgraded the OS (Snow Leopard -> Mtn Lion) on the SSD and would like to have the old OS around so I can boot into it for s/w development and testing.

PS: Sorry, I don't have the error the machine reports when both drives are loaded and I try to boot in single user mode (or any mode). It's a pain to mount the HDD in the CD drive bay. When I get a chance to tear down my system and go through that h/w configuration, I will note the error and edit this post with the exact details. Perhaps someone knows what might be wrong with just this info, however. Thank you.

1 Answer 1


I had a similar problem once under Windows where the drive got the same drive identifier (UUID). Googling for "macos change UUID" led to a superuser thread re MacOS 10.6. The UUIDs are assigned to partitions and you can view them interactively from the disk-util, selecting a partition and pressing the info (i) button near the top of the disk-util window. You'll also see the disk identifier there (e.g. disk0s2).

Looking at the other question, the command then be

sudo /System/Library/Filesystems/hfs.fs/hfs.util -s disk0s2

But I didn't try that here, so enter at your own risk (and with your own disk identifier).

However, there is a program named SuperDuper which I use for this purpose (it creates exact working clones of drives and I have my systems back to OS X Tiger still around this way), but I'm not sure if you're saying you upgraded to Mountain Lion already. If not, you could clone the disk that way and then upgrade.

  • Thanks. Next chance I get to reinstall the drive, I'll investigate this and review the other thread - it looks promising. I'll post my results once I do. Also, I have already upgraded to Mt Lion on the SSD, so the drives have diverged significantly from a Data perspective. I don't think SuperDuper is an option at this point. Thank you for the suggestion. Commented Oct 8, 2012 at 22:18
  • This worked, thank you. (tried to upvote your comment, but I don't have rep pts, yet to do it). I used diskutil and hfs.util -k to verify that both UUIDs were the same (hooked one drive up to a USB external drive in order to check it). hfs.util -s changed the UUID on one of the drives (be sure to have the drive unmounted and make sure all drives with UUIDs are changed). Commented Oct 9, 2012 at 23:18
  • I wrote a detailed blog post about adding an SSD to my Macbook Pro which includes information on the need to change the UUIDs for an exactly duplicated drive and how to do it. Commented Oct 10, 2012 at 0:18
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    @AndrewPhilips the problem was that internally the volumes (formatted partitions) are identified by their id and these need to be unique system wide. They are not really the UUID you see in Apple tools. The gory details are in my answer to superuser.com/questions/344706/… Changing the "UUID"s of the volumes fixed it (made them system wide unique). However there are other UUIDs (Apple calls them "Media UUID"s) that identify the partitions themselves. They currently only need to be unique within the same disk. Commented Dec 2, 2014 at 23:09

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