How does Mac OS X decide which volumes to mount? One of my disks doesn't mount at boot 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.
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:


Select the volume in Disk Utility
Choose File->Enable journaling (if it is greyed out, journaling is enabled.
Verify by choosing File->Get info; you should see "Mac OS Extended (Journaled)" as the filesystem type.