I have a Seagate Canvio 500 GB USB 3.0 external harddrive (power comes from USB 3.0, no extra adapter), it has given me no problems up to now, and i'm fairly sure the problem is actually Windows 7 Ultimate 64 Bit/My doing.
After a few minutes of not being used, it would automatically power off like its supposed to. I dont know when it stopped powering off on its own, but it probably happened when I was networking the drive so my brother could access the movies/music off of it without unplugging it from my comp and plugging it into his laptop.
Ive disabled all share settings and such to try to get it to start powering off on its own again but have had no luck. Even tried disabling search indexer, but hasnt helped either.