What Mac and what version of OS X are you running?
Does that Iomega drive have it's own power supply or is it drawing power down the USB cord?
If it is, then there is a possibility that the iPod and the Iomega combined, are drawing too much power.
Get a power adapter for the Iomega
Get a USB hub and plug the devices into it.
If you have an iMac. Shut it down, disconnect ALL peripherals except the keyboard and mouse.
Remove the power cord.
Wait 5 minutes
Plug it back in and boot it up.
Leave the Iomega disconnected for now. Connect the iPod and see if your iPod reappears in iTunes.
I have an ipod nano and a mac with an extra iomega external HD. I have most of my music stored on the HD, but not the iTunes software (so there shouldn't be an issue with duplicate software).
When I tried to switch out music today, I plugged in the iPod (External HD was also connected), it didn't show up on my hard drive OR iTunes. When I disconnect my iPod, it says I have to eject it before disconnecting. The problem is that I don't have anything to disconnect! It's not showing up! I think this is an issue with my external HD because when I unplugged my ipod, the Iomega HD disk image disappeared and got an audio, improper disconnect message (weird. beeping).
So I disconnected/ejected my Ext. HD and restarted to see if it would fix the issue, but it didn't. Now I can't use my iPod because it says that I'm connected to my computer (even when I'm not) Argh. Anyone have any ideas?