The Toshiba USB external hard drive described below is apparently not a good fit for a Mac using Time Machine. It goes into sleep mode on its own and will not wake up for its regular backup duties. I tried every configuration setting I could find, without success. It's no doubt a great hard drive otherwise, but not for use with Time Machine. Here's the drive that didn't work well with Time Machine: Toshiba Canvio 1.0 TB USB 3.0 Basics Portable Hard Drive - HDTB110XK3BA I sent back the Toshiba drive and got a comparable Seagate external drive. It works without any problems. I have a Mac Mini (late 2012) running Mountain Lion 10.8.2.