Mysterious external hard drive problem!

I have a 500GB Toshiba hard drive with a backup of my Macbook on it. I can't remember how I last formatted the disk--it might be partitioned, it might just be a Time Machine, it may or may not be PC-compatible, I don't remember. Anyway, my actual computer's hard drive is busted and now that I'm trying to access the backup I can't get this one to show up on any computers: an iMac doesn't recognize it at all (even in Disk Utility), a Dell says there is new hardware connected but nothing shows up in My Computer. The hard drive whirs and the light goes on but it is white and not blue. Suggestions? Is there some software I can download that will see the hard drive? I JUST WANT MY DATA BACK

It sounds like your Toshiba HD is not working if Disk Utility can't see it and the Dell can't access it. You can try opening Disk Manager on the Dell and if the drive appears, assign it a drive letter. After that it should show up in "My Computer" but that doesn't mean your data will be accessible.