SUMMARY: - When I plug my Aperture disk into my iMac, the filesystem is not navigable, nor is the disk unmountable. - When I plug my Aperture disk into my macbook, everything seems to be fine. LONG VERSION: I have a pair of Seagate external hard drives. One is my time machine drive that we'll call "Backup" and the other is my "Aperture" drive. Both are connected to my aluminum iMac. Earlier today, I was having some problem with the Backup drive. It didn't want to mount. So, I logged out and back in. No dice. Then, I noticed that the Aperture drive wouldn't unmount. I tried again and again to repair both of these disks and nothing would work. I unplugged the Aperture disk. Now, I could repair and work with the Backup disk just fine. No problems. So, I plugged the Aperture disk back in. Everything went ugly on me. For grins, I booted the iMac from the CD and ran disk repair and permissions repair. Then I booted into single user mode and ran fsck -- just for good measure. Just wanted to make sure that the iMac's HD and filesystem were A-OK. Meanwhile, I plugged the Aperture disk into my macbook. Ran disk repair. No problems. I plugged the Backup disk into my macbook. No problems. Now I plugged them both into my iMac -- ugliness. I unplugged the Aperture disk and the Backup disk now worked fine. I unplugged the backup disk, and plugged the Aperture disk in. Aperture disk would not work. As I type this, everything is fine with my iMac and the Backup disk. In addition, I have the Aperture disk plugged into my macbook and everything seems to be fine. Anyone have any ideas? Thanks, as always!