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Found a good deal on an external 750gig Maxtor drive today and picked it up to mount to the iMac (firewire). First of all, if you are about to do something similar, just pass on the Maxtors and buy something else. Will save yourself a lot of trouble.

However, if you already bought it and are in the same boat as I was here is a fix:

The drive would not show up in finder but it would be in Disk Utility. I could not format or repartition the drive no matter what angle I tried, and always got a response of "could not unmount disk". The short fix to this is you have to manually find out where it is mounted and unmount it via the console before you can then repartition (GUID) and reformat (Mac OS) the drive. A quick googling found that everyone has this problem, and Maxtor just seems to be a company who can't simply put out a simple external drive but they have to throw a lot of extra crap into the mix.

Try doing the format via disk utility and get your error. Then open up a terminal, get to root by 'sudo su -' and then run dmesg. Look for an error like such:

0 [Level 3] [ReadUID 0] [Facility com.apple.system.fs] [DevNode /dev/disk2s1] [MountPt /Volumes/Untitled 3] [Path /Volumes/Untitled 3/.Spotlight-V100/Store-V1/Stores/7C3E0BE7-2F06-4CC2-B368-59D0E0604211/indexState] [FSLogMsgID 1667112663] [FSLogMsgOrder Last]

In my case, it was mounted in /Volumes/Untitled\ 3 but was hidden there.

Force the unmount:

umount -f /Volumes/Untitled\ 3

Then go back to Disk Utility and repartition; reformat.

I put this out there in hopes that google will help someone else find this answer quicker than I was able to.

Cheers!