I have an external hard drive that does not mount to my Mac laptop, nor is it visible in Disk Utility. It is, however, visible in the System Profiler under the USB section. Any suggestions as to how I can get this drive to mount?
The only theory I can imagine for what you're describing is if you're booting into Safe Mode and if you are, Lion won't mount your 'unsafe' drives or even recognise them (in some instances).
nb. OS X will still write hidden dot files (.fsevents .DS_store .Trashes etc) to those unmounted drives / partitions / filesystems.
Although this theory is unlikely (as you'd surely notice the lengthy boot time - during which OS X rebuilds all the cache / kexts / preference files), you can find out whether you're booting into Safe Mode (and further investigate your issue) by opening the Terminal and sudo'ing to root.
To determine your boot parameters, type:
Look for the "boot-args" field (which may not be there, in which case you aren't booting into Safe Mode) and if you have any of these listed, it means:
You can add any of these to your nvram settings if you wish, and they'll be persistent (until you remove or adjust them) - this example below would boot you into Safe Mode with Verbose output indefinitely:
By design, you can safely clear your entire nvram settings and the system should rebuild upon next boot. To clear your nvram on the command line, type:
As for OS X recognising your drives, there is so much more information available using the Terminal. The diskutil command is very easy to use and has simple, clear instructions and output. Type something like:
and you'll see all disks / drives / partitions (mounted and unmounted) listed.
To see all your mounted filesystems (and their mount-points), simply type:
You can mount your EFI system partition by creating a directory to use as a mountpoint:
then mount the 'hidden' system partition
(-t is the filesystem, -o denotes mount params [rw is read/write, use ro for readonly])
and to unmount
diskutil and hdiutil commands give you a lot of information which will almost certainly give you enough information to work out what's wrong with the unmountable drive.
some drives require that a utility provided by the manufacturer be installed on the system to authenticate the user; this should only be the case if you used said utility to encrypt or otherwise password protect the drive.
I've just experienced a similar problem. I have a WD 2TB dual drive, which suddenly wouldn't mount. I could see it in the System Profiler, but it was greyed out in Disk Utility. The problem wasn't the ports or the cable, and repairing the disk via Disk Utility wasn't working.
But Disk Warrior fixed it.
DiskWarrior recovers your files including your photos, music and videos. DiskWarrior cures those cryptic errors reported by Disk Utility. DiskWarrior uses a different approach to disk directory repair than other programs.