Hi there, what if you try to change the cable or look for the drive on the Mac on disk utility (Go> Utilities> Disk Utility) or on the PC under disk management (Right-click computer/my computer> Manage> Disk Management)?
I tried looking as you advise on both the mac and PC, but no sign of the external HD. I will try a new cable, but it seems the cable is connecting OK as the HD lights up as if its registering being connected - if that makes sense?