How does Mac OS X decide which volumes to mount at boot?

Discussion in 'macOS' started by ajbrehm, Jan 11, 2012.

  1. ajbrehm macrumors 6502

    Aug 14, 2002
    Zurich, Switzerland
    I have four disks in my Mac Pro. The boot disk and two others mount when OS X starts but the fourth one does not. I can mount it in Disk Utility and it appears to be all right.

    How can I tell Mac OS X to mount it?

    How does Mac OS X decide which volumes to mount at boot? Where is that done or configured?
  2. Heynonny macrumors newbie

    Jan 4, 2012
    You say "in" but if the fourth drive is actually an external then the Finder Preferences must have "external disks" checked under"Show these items on the desktop"
  3. ajbrehm thread starter macrumors 6502

    Aug 14, 2002
    Zurich, Switzerland
    No, all four drives are internal.

    Also, this is about mounting the drives, not about displaying them on the desktop (or the Finder).
  4. Shadow%20Mac macrumors 6502

    Dec 28, 2007
    I've wondered about this for some time now. Perhaps its a setting from disk utility? No idea... I could see it being something like a checkbox for "mount this disk on startup"

    Sorry I couldn't be more helpful!

    Any other answers are appreciated, I am having a similar dilemma with an internal SSD and an internal HDD. Neither of them are the boot volume, but the SSD mounts on startup while the HDD does not.
  5. ajbrehm thread starter macrumors 6502

    Aug 14, 2002
    Zurich, Switzerland
    I have also asked this question at the Apple Stackexchange but all I got was the Finder display icon thing and a nasty discussion with some angry guy.

    I will make sure to post here when I get an answer elsewhere. In the mean time, everybody else reading here, please help us! :)
  6. ScoobyMcDoo macrumors 65816

    Nov 26, 2007
    Austin, TX
    You might try adding an entry to /etc/fstab. You can find documentation by opening a terminal and typing "man fstab"
  7. ajbrehm thread starter macrumors 6502

    Aug 14, 2002
    Zurich, Switzerland
    Actually, that might solve ShadowMac's problem. I found a site that explains this a bit (although for people who want to stop a drive from mounting):

    Or this might:

    But it does not answer my question. I need to know how Mac OS X decides which drives to automount at boot. Mac OS X obviously doesn't rely on fstab to make that decision.

  8. NeilSmithline macrumors newbie

    Jan 26, 2012
    Thank you ScoobyMcDoo

    I didn't know that MacOSX used fstabs. It has that fstab.hd file that says its deprecated but no fstab by default. Once you pointed me to the man page, and after hitting myself in the forehead for having not looked at it before, it only took me 10 minutes or so to figure it out.

    I still don't know the answer the original question about how MacOSX determines what to mount where. But I do know how to work around it.

    I like to have my Applications directory on a separate partition. That makes life easier when I FUBAR my / filesystem as I don't have to install all my apps again. I've tried symlinks and such but whatever I try never quite works right. But now my fstab reads:
    And life is good.

    The first line tells MacOSX to not automatically mount disk0s5 to /Volumes. The second line tells it where I want it mounted. Being it is a rather important directory for the OS to work, I use the "union" option so that the original files are still accessible from /Applications. Big win should my mount fail.

    Thanks all!


    PS: In case you're wondering, I routinely FUBAR my / partition. I find that my computer always *needs* just one more tweak and sometimes that is a tweak too far :D
  9. larkost macrumors 6502a

    Oct 13, 2007
    By default MacOS X mounts everything that it can. So there is something that is keeping your final disk from mounting. My guess is that there is probably something in syslog that will tell you what it is thinking. There might be something in /var/log/system.log, but I would go the full route and use this right after you boot:
    sudo syslog -B
    That will get you all of the messages since the machine booted. Usually there is a bit of extra debug spew in there that does not make it into any of the text log files.
  10. sososowhat macrumors 6502

    Feb 20, 2003
    Palo Alto, CA
    Tinker Tool seems to be the way to go. Under 10.8.2 I edited /etc/fstab as described to prevent mounting & it didn't seem to work. Tinker tool launched & thought the mount was prevented, but it wasn't. I deleted that line in TinkerTool, then re-added it using the tool, and voila, my old 750Gig HD (in the optical bay of my MBP doesn't mount when I boot) and I silent and still off the SSD. Happy camper. Thanks for the pointer.
  11. rthpjm macrumors 6502a


    Jan 31, 2011
    I know this is an old thread, but the OP's question was not really answered (or it was, but without understanding why!).

    It is diskarbitrationd that manages the system view of mounting disks. Here is the man page description:

    More importantly the man page explicitly states that diskarbitrationd reads the /etc/fstab file:

    Therefore the answers given for using /etc/fstab are correct, and this is the reason why.

    You can read the manual page for diskarbitrationd in the Terminal:


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