My iMac (mid-2010 model, Snow Leopard) has a problem with S.M.A.R.T status: Failing and it has become read-only. Because I didn't want to waste money on the repair, I bought an external HD, so the internal HD is now unnecessary. How can I disable mounting the internal Macintosh HD at boot?

  • 2
    Your iMac is almost certainly still under warranty, so getting it fixed will only cost you a trip to the nearest Apple Store. – user5516 Oct 27 '11 at 9:37
  • I'd also like to know the answer to this. I have an out-of-warranty iMac (2007) whose internal drive died. Instead of replacing it, I am now booting from an external (FW800) SSD. However, the iMac's internal drive goes into clicky-click mode from time to time, even though I deleted all volumes from it and ejected it in Disk Utility. I'd really like a way to make it completely invisible to the OS, just as if I had physically disconnected it. Maybe there's a way to fiddle in the EFI configuration? Starting a bounty for this question! – Pascal Bourque Jul 4 '12 at 17:39

This is quite old, but still works perfectly under Lion, I'm using it for several partitions: http://hints.macworld.com/article.php?story=20060930150059172

Fast path:

  1. find the Macintosh HD volume UUID: while your Macintosh HD partition is mounted, open Terminal.app and type diskutil info /Volumes/Macintosh\ HD, then read the line beginning with Volume UUID
  2. edit /etc/fstab as root (I personally use vi but any editor will do, the article even mentions pico! If you have TextWrangler installed - you should - with the command line helpers, sudo edit /etc/fstab will work too)
  3. add a line like this one: UUID=F0E430C1-5558-3BB3-9FA9-6904B663FEEA none hfs rw,no auto of course replacing the uuid with yours.
  • It seems like this would prevent auto-mounting a volume at boot, but not hide the physical hard drive from the OS. In my case, I have erased all volumes from the defective internal hard drive and want the drive to stay off forever, just as if it had been physically disconnected. As I type this, I hear the iMac's internal drive click like there's an army of hamsters running on its platters. I want to make this stop, and it seems like unmounting and deleting all volumes is not enough... – Pascal Bourque Jul 4 '12 at 23:10
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    The OP title is clearly about preventing a partition to mount. You are trying to do something totally different here: disable a sata device from being seen, which only can be done in software at the driver/ioreg level. I'm not sure you can do that in a way that would be easier than open the machine and unplug the cable. – cdelacroix Jul 4 '12 at 23:55
  • blush You are right... It looks like I hijacked a question that is not exactly what I need. The OP and I are in the same situation: having a failed internal drive, booting from an external drive, and wanting to get rid of the internal drive. However, his question is indeed specifically about how to disable auto-mounting the drive at boot. That'll teach me to read carefully. – Pascal Bourque Jul 5 '12 at 1:05
  • Is there any alternative to running diskutil to find the UUID? It fails with an error about dependencies missing and says not to use it in single user mode. My machine is not bootable, even from an external hard drive, because the internal one causes the entire system to beachball almost continuously. – Abhi Beckert Apr 16 '13 at 22:49

I'm not sure if it works with external drives, but the "normal" way to change your start drive is this:


  • It does work on external drives (USB or Firewire or no doubt Thunderbolt) but I don't think that's what the question was about. I think the question was, even after starting up from an external drive, how does he prevent his now useless internal drive from mounting/showing. I have no clue how to do that. However, I'd go for comment #1 which is to take the iMac to an Apple store or call Apple and have the drive replaced under warranty. – Richard Oct 27 '11 at 17:20

I know this isn't what you've asked, but why don't you transplant the working external drive (which is no doubt SATA) and replace the faulty internal drive. It should work directly, basically plug and play, ermmm boot.

  • I was going to suggest the same thing. The easiest way to disable the drive entirely would be to open up the iMac and unplug the dead drive, but at that point you may as well take the external drive out of its casing and install it in the iMac. – iynque Jul 9 '12 at 0:12
  • Unplugging the internal hard drive is not "easy", that's a 30 minute process requiring suction cups and torx screwdrivers and voiding your warranty. – Abhi Beckert Apr 16 '13 at 22:42
  • I never claimed it was easy. Perhaps some, maybe even you, might struggle with this procedure, but it would appear that the OP knows what he's doing. His knowledge of S.M.A.R.T is a clue that he's likely able to perform such a replacement. I answered in consideration of the OP's ability, not yours. As such, your down vote is quite unwarranted. – Alexander - Reinstate Monica Apr 17 '13 at 1:52

I'd also recommend you try to use my util to auto disable your second drive on booting OS with an opportunity to enable it in one command whenever after. Please check it out: https://github.com/igorleanch/2ndrive