These drives are known for connectivity issues. This is what I end up doing. Put the power plug in the hard drive and listen to it start up then put the firewire or usb plug in and if that doesn't work do it in the opposite order e.g. plug the usb or firewire plug in and then the power. Eventually i get the drive to mount but once I do I try real hard not to disconnect it. This seems pointless I know. Happily I see that WD drives have mounting problems on PCs as well.