My external hard drive stopped mounting

My external hard drive stopped mounting on my MacBook. When I ran Time Machine the other night (for the 1st time in a few months) it appeared the usb cable was sensitive to touch ... Meaning it had to be just right to make connection. But I believe everything was backed up. The next day when I went to check on it I realized the HD was no longer mounting. After I did a permissions repair it is now not recognized in Disk Utility (it was prior). Today I checked my other external HD, which was my 1st backup, and it also does not mount ... Nor does it appear in Disk Utility. When I hook them both to a PC they are recognized. Any ideas on what has happened?