Try a different usb cable.

Plug the cable into the back of the computer not the front usb ports.

Shut down your computer and then plug the usb drive in then restart your computer.

Sometimes these things help sometimes not.  this is a common problem with Windows 7.  Sometimes there is no fix.

Did you try installing this software from Seagate on you computers?  Its compatible with W 7 and Vista both 32 and 64 bit.