I have the same problem:
I own a SimpleTech external hard disk(SignatureMini 320GB) and I've used it before in Windows 7.
but recently when I plug it to my laptop (Windows 7), It's not listed in My Computer.
there's no problem with the hard (I've tested it on other computers) and also I can read data via softwares like Paragorn Hard disk Manager, so the drive is accessible even in Windows 7!
There isn't any "USB Device not recognized" error, but 7 doesn't show it's partition!
any solution?