Why can't I see my external hard drive, Intel iMac / 500 GB, G-Drive

Why can't I see my external hard drive? I have a new iMac, Intel based i5, running Lion (2-3 months old). I have a 500 GB G-Drive external hard drive that has showed up when connected to my iMac for the past 2-3 months. Now it's gone. It's NOT visible when using Disk Utilities. I have all of the radio buttons chosen in the Finder Preferences. I've unplugged and replugged in the drive, have switched the cable ports and have switched firewire cable. The G-Drive powers up and whirls, so I don't believe the problem is with the drive. Yes, I set my Preferences to make sure ALL disk types are visible. I've restarted my iMac on a number of occations. I rest my SMC by totally unplugging my iMac for a minute, then plugging it back in. Any suggestions?