You have to be a bit careful here as the Iomega model a friend bought contained 2x500gig drives spanned using some sort of raid configuration. The disk format as supplied was Fat32 which is compatible with Mac and Windows. However the literature offered the option to convert to the current Mac file structure.
Unfortunately my friend being a Mac person wasn't sure it was working (I'd failed to warn him that it would take hours) and decided to turn it off when he went to bed, meaning to resume the formatting in the morning. Which unsurprisingly royally messed it up.
All I could do was restore the two disks separately as 500 gig drives using Fat 32.
In the end he dumped the box and used the drives singly in a SATA dock..