not mounting external drive

Try resettng both the SMC and PRAM, it won't hurt to do each 2x.

SMC RESET

• Shut down the computer.
• Unplug the computer's power cord and all peripherals.
• Wait 15 seconds.
• Attach the computers power cable.
• Wait another 5 seconds and press the power button to turn on the computer.

PRAM RESET

• Shut down the computer.
• Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
• Turn on the computer.
• Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
• Hold the keys down until the computer restarts and you hear the startup sound for the second time.
• Release the keys.

Your profile indicates you are running 10.6.8. Is it possible you upgraded to Lion and just need to set your Finder preferences to show external disks on the desktop? From the Finder pull down the File menu and select preferences, then make sure that you have a check mark next to the 'External disks' open in the 'Show these items on the desktop:' section.

I also suggest launching the Disk Utility application that is located in the Applications/Utilities folder. Does it show that your external hard drive is connected to your system? If not, check your cabling and the connections at the computer and hard drive.

Your welcome.

This worked for me: With computer on and drive off, plug in drive to computer. After a moment's wait, turn on the drive.

In my case the drive then appeared on the desktop.