I bought an external drive that I planned to use with time machine on multiple computers. It is a 500 gig... and is usually connected to my iMac. I connected it to my MBP to try to back it up, and the MBP has been sitting there with the drive connected for 3 hours without backing up once. Under SysPrfs, it says TM is on, and that the disk in use is the 500 gig HD. What do I do? Thanks.