My Simple Tech 320 Gb external hard drive wasn't backing up as usual & the backup utility wouldn't recognize it, iMac dual-core Intel, OS 10.6. Tried re-booting, checked the power switch, transformer, etc. & found that the drive wasn't getting power. The 12 v. d.c. power supply plug wasn't making contact with the center post. Used a jewelers screwdriver to bend out the contacts. Success.