You could try using another USB port on your computer. Perhaps that USB port you have the IOmega external harddrive plugged into is bad.
More than likely not, though.
The problem is usually inside the IOmega's case. (External enclosure)
Inside that IOmega external harddrive's case is a regular harddrive as used in a desktop, or laptop computer.
Harddrives are physically rated in width, as well as type, and size.
A laptop harddrive is 2.5 inches across in width. (2 and a half inches)
A desktop harddrive is 3.5 inches.
The larger desktop harddrives are usually used in external harddrives that are 500GB, and larger.
(Not always the case when it comes to the 500GB size)
Harddrives have two technologies, or types.
IDE (PATA) or SATA.
You didn't state the Model Number so I can't even hazard a guess at this point.
The harddrive inside the case plugs into an Interface.
The Interface may be separate, and connected via a cable to a small circuit board, or the Interface may be soldered directly to the small circuit board.
It is this Interface, and circuit board which go bad.
The solution is to remove the harddrive inside, purchase an inexpensive external enclosure, and install the harddrive into it.
The external enclosure has a USB cable just like the IOmega external harddrive did.
As stated the harddrive inside is just a normal harddrive. Made by a harddrive manufacturer.
Could be a Western Digital, or Seagate, or Hitachi, or Maxtor, or Fujitsu, or IBM, just to name a few.
(Seagate bought out Maxtor)
On the harddrive will be a decal with it's specifications.
You will also be able to physically measure the width.
This external hookup will support harddrives that are 2.5, or 3.5 inches in width, (And 5.25), plus will support both IDE (PATA) or SATA harddrives,http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=7220658&CatId=3770
This external enclosure is for a desktop sized harddrive, 3.5 inches, and will support either IDE (PATA) or SATA harddrives,http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=1951866&CatId=2780
This external enclosure example is for laptop sized harddrives, 2.5 inches, and will support either IDE, (PATA), or SATA harddrives,http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=7041055&CatId=2783
Just one example of how to disassemble an IOmega Select Portable harddrive,http://www.youtube.com/watch?v=NvUvz5Qb4vw
Other styles are listed in the Suggestions column down on the right.
Not a good source for information on disassembling an IOmega external harddrive, but there is very little info available.
(It isn't that tough for any external harddrive enclosure. The screws are usually hidden under labels, and the case pops apart in two halves. Just a little different design here, and there)
For additional questions please post in a Comment.