First time using external hard drive for backup of computer....

Was this drive purchased formatted for Mac OS X? Has it been used before? If it was formatted for PC then it needs to be reformatted for Mac OS X..

Exactly what HD is it, chances are it is not formatted to work with your iMac however you can easily fix that. In Finder right click the EHD's icon and select Get Info and look at the format type. If it's formatted for OS X it will say Mac OS Extended (Journaled), if it says anything else open Disk Utililty ( Applications - Utilities - Disk Utility) select the HD and then click the Format Tab and Format for OS X. The whole process should take less than 5 minutes.

Do not use the Lacie utilities, use Disk Utility to format.

To use Disk Utilities, attach the drive, open Finder > Applications > Utilities > Disk Utilities and then highlight the LaCie drive in the left panel of the DU window. In the right panel, choose the Partition button top center of the pane. Then decide how many partitions you want on the drive, one is just fine.

Then choose the format, the default is Mac OS Extended (Journaled) which is prefered.

Then name the partition or drive.

Then choose the partition table, the default is GUID which is prefered.

Then Apply and you will get to confirm that you want to erase and format the drive.

Quit DU and you are ready to go.

That is correct, both should show up on the left hand side .

Be sure you choose LaCie and DO NOT CHOOSE MAC HD.

Cheers

Don

Yes, the external drive is the one it should be referring to if you highlight that drive as the one to format. You can always see in the left panel which drive is highlighted. Just take your time and double check yourself along the way.