This might be so obvious it’s not actually a tip, but I just learned it today. This is useful if you have more than one hard disk in your Mac OS X system. With only one disk (we’re ignoring disk partitions here), it’s impossible to unmount the disk, because you can’t make your startup disk unavailable to the system.
With more than one disk (or partition), it’s occasionally useful to unmount a volume, either to “hide” it from some activity or utility you’re using, to run a disk checking utility on the unmounted volume, or just to keep down the clutter on your desktop.
Way back in the day, when SCSI hard disks were all that the Mac could access, I used a control panel called SCSIProbe to mount disks which, for whatever reason, were not mounted and available on my desktop. These days I’m using Mac OS X full time, so SCSIProbe is no longer available. And SCSIProbe was only good for SCSI disks. Since I recently decommissioned the one SCSI disk in my system, in favor of a pair of ATA (IDE) disks, I need a new tool anyway.
There’s a command line tool to mount and unmount disks, but if you want something easier, just use Disk Utility, provided by Apple, in your /Applications/Utilities directory. After launching it, select the disk volume in the list on the left, and choose Mount from the Options menu. Simple!
Note that you have to select the disk volume, which will appear as the disk name indented under the physical disk item, which just lists the disk’s physical capacity. If you select the physical disk, the Mount command will be dimmed out. If your disk has multiple partitions, they’ll all appear under the physical disk item, and you can mount them in the same way.