Firewire Owc 800 Hard Drive


I'm running 2 mac's.

A G4 with OS9.2 and an iMac with OSX.4

I unmounted my Firewire drive so that I could plug into my imac to download some files. When I plugged into the imac the hard drive would not appear. So I unplugged it and put it back on the G4 and now it won't appear on it either. It gives a message "The alias Serial Port Monitor could not be opened, because the disk "Mercury", which contains the original item, could not be found.

Mercury is the name of my hard drive that won't mount.

Any ideas?