Hey there Paul and welcome to the community!
As there's an issue with a drive, of course the first step should be (as usual) to backup any important data which you might have on it, just to be on the safe side. After that I'd recommend that you check the drive with DLG (Data Lifeguard Diagnostics). Run both test (Quick and Extended) to make sure that everything is OK with the HDD itself. Here's how to do it:
How to test a drive for problems using Data Lifeguard Diagnostics for Windows. I'd also recommend that you try a different external drive with the Windows 7 computer if possible, to see if the same thing happens. You could reinstall its USB driver and try to connect the HDD again. Try all the USB ports as well.
Hope that helps. Please let me know how everything goes.
Boogieman_WD