External hard drives not working in Windows 7

I had an Iomega 80gb from my XP computer and an AData 500gb from my windows 7 laptop plugged in my windows 7 desktop and they worked just fine. One day I went to retrieve some data from both of them, only to find, that the same error messages popped up for both of them. The first one for both said I need to format it. The second message says the the drive is not accessible, but to make sure the right drivers are loaded and that the drive is not corrupted. Furthermore, I tried the drives on other computers and the same messages are given.
I did reformat the AData drive and lost everything, which is not that bad because it was backup for my backup. However the older Iomega drive has stuff on it that I would like to recover. Does anyone have any answers:confused:
Thanks