Hi, I am going to assume that it worked on both of them before is that correct?
If so one quick thing to try is plug it into a Windows box and listen to see if you hear any loud grinding noises, etc. if you do hear a grinding type noise the hard drive is on its way out.
If not, Open > Disk Management (Run > diskmgmt.msc > Enter) and see if it is recognized in Windows that way (generally with all other External HDDS unplugged it is the last in the list) If you are unsure look down in the system try for the icon to safely remove the drive (if its there go ahead and do it and watch the diskmanagement screen to see which drive drops out). There are some command prompt commands you can run if the drive is recognized in windows but its just not showing up in my computer.
Also, just as a sidenote when moving a drive from a Windows box to a Mac box it is best to do the safely remove way on Win and Eject on Mac. This can prevent many I/O errors and data corruption.
One last thing is do not try and save anything to the drive until you are a 100% it is fine if you get it working.
If your not a novice you can try PhotoResc(its for more than photos) for Windows and try and recover the data that way but you need some place to save it all and it takes quite a bit of time. (Use this very cautiously.)