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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?

diskutil list /dev/disk2s2
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk2
   1:                        EFI                         209.7 MB   disk2s1
   2:                  Apple_HFS Broken                  999.9 GB   disk2s2

Update: There seems to have been a misunderstanding about this question. I do not want to find out what's wrong' with my disk I named "Broken". It appears to work fine. I also reformatted it several times. I want to know how to tell OS X to mount a specific disk at boot. I named the disk "Broken" because it doesn't mount automatically, not because I have any problems with it as such and nor are there any error messages. Just forget I mentioned the disk and answer the question how to tell OS X which disks to mount at boot, if you know the answer. Thanks.

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You say you can mount it in Disk Utility as if you cannot mount it via the Finder sidebar? cl.ly/DC0k –  gentmatt Jan 9 '12 at 18:40
    
How would I mount it via the Finder sidebar? –  Andrew J. Brehm Jan 9 '12 at 19:28
    
Did you click on the link of the first comment? –  gentmatt Jan 9 '12 at 19:29
    
Yes. But my Finder is configured to show all hard disks. Don't know how to make the Finder mount disks though. Your screenshot doesn't show that. –  Andrew J. Brehm Jan 9 '12 at 19:32
    
Why do you need it to be auto mounted upon start? I mounts quickly as soon as you navigate there in finder. –  gentmatt Jan 9 '12 at 19:40
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4 Answers

OS X uses diskarbitrationd which discovers new storage devices and probes them for mountable filesystems. The Disk Arbitration framework handles notifying applications of disk mount/unmount events, and allows them to influence whether a volume is mounted or not.

Consult the man page for diskarbitrationd for very limited further information; for example, diskarbitrationd consults /etc/fstab to determine if the discovered filesystem should be mounted other than at the default location(/Volumes/) or with special options (or not mounted at all.)

Reasons for diskarbitrationd not mounting a filesystem or "volume" on a device may include:

  • Errors (corruption) in the partition table.
  • A mismatch between the partition type and the filesystem.
  • A damaged filesystem which fsck cannot repair.
  • Hardware failure.
  • Filesystem is listed in /etc/fstab with noauto.
  • An application has used the Disk Arbitration framework to prevent auto-mounting

According to your comments, all your journaled filesystems are mounting, and the filesystem which isn't mounting doesn't have journaling turned on. Journaling was introduced in 10.2 Server and 10.3 Client and by now is considered a vital, critical feature. It should be enabled unless you really know what you're doing; hence why you must hold down the option-key in Disk Utility to disable it in modern versions of OS X. Make sure the volume has journaling enabled:

  1. Select the volume in Disk Utility
  2. Choose File->Enable journaling (if it is greyed out, journaling is enabled.
  3. Verify by choosing File->Get info; you should see "Mac OS Extended (Journaled)" as the filesystem type.
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The file system is clean and the volume has been formatted twice. I can also mount the volume manually both in Disk Utility and in the command line. –  Andrew J. Brehm Jan 10 '12 at 12:17
    
Please provide the diagnostics I asked for - the output from Get Info on the volume which won't automount, output of mount once it has been mounted, diskutil list (the partition list) of the drive, and diskutil activity output during mounting. –  BeeDee Jan 10 '12 at 15:02
    
Yes, yes, yes, wait until I get home... Now. –  Andrew J. Brehm Jan 10 '12 at 17:23
1  
I've flagged the question for moderator review and asked that it be closed since you've refused to provide the necessary information. –  BeeDee Jan 10 '12 at 18:38
2  
Sometimes there are questions with insufficient detail and the asker never comes back, so they're closed. However, in this case, Andrew seems to be actively responding, so I'm going to leave the question open. He seems invested enough to realize that omitting troubleshooting steps may result in not getting an answer. –  Kyle Cronin Jan 10 '12 at 19:56
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Assuming there's no dramas with the volumes themselves, you can mount additional volumes at login by adding them as Login Items for your user account in the System Preferences.

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That opens a new finder window every login. No matter if you check-mark 'hide'. Kind of annoying. –  gentmatt Jan 10 '12 at 6:58
    
It's also impossible since Login Items are opened, not mounted. How would Finder even know which device to mount for an icon named "Hard Disk 2"? –  Andrew J. Brehm Jan 10 '12 at 8:30
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In my experience, it doesn't open Finder, it just mounts the Volume. I use this to mount shares from my Drobo (although I admit that I'm assuming that this works the same for local volumes). Will confirm the steps when I get home. –  Tony Johnson Jan 10 '12 at 9:17
    
Volumes can be added to startup items by going to System Preferences > Users & Groups > Login Items. Select "+" to add a new item and select the Volume in question. Have confirmed that this works with Local Volumes as well as Network Shares. The result is the Volume mounted and an icon on the desktop. No finder windows are opened. –  Tony Johnson Jan 10 '12 at 10:12
    
So this will really mount the volume, not just open (the already mounted) volume? –  Andrew J. Brehm Jan 10 '12 at 12:17
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up vote 1 down vote accepted

The volume is mounted automatically again.

I never did find out how Mac OS X decides which volumes to mount and which not. But whatever it is, Mac OS decided to mount the volume again. Apparently the solution is to reboot until it works.

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I have another disk now which didn't auto mount but mounted fine manually. –  Andrew J. Brehm Apr 21 '12 at 14:27
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Go to System Preferences>Accounts>Login Items. Click on the plus sign and add your drive on that list. Every time you log into your account, Finder will automatically mount that drive.

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This is the same answer as Tony Johnson's. Read the comment there. –  Andrew J. Brehm Jun 7 '13 at 7:29
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