I just thought I'd make a post about something I found out recently about Mountain Lion and a fair number of external hard drives: A lot of them won't work! I'm in Herndon Va. and we work with a company name Software and Computer Systems Company, LLC out of Reston Va because they sold our company a suite of drive testing (Scannerz) and security (FSE) tools. I know Scannerz has been mentioned before on this forum before. In any case, since they're obliged to answer my questions as a customer, I had a client that updated to Mountain Lion and used a Seagate external drive for backup and all of a sudden what was working fine under Snow Leopard is now extraordinarily problematic. After contacting SCSC, they directed me to the Seagate website, and there were TONS of listings of problems with external hard drives. The worst offender is what I'd call a "narcoleptic drive." It just decides to take a nap, right in the middle of operations - it literally goes to sleep during a read or write, leaving the end user with spinning beach balls. This problem can occur on both FireWire and USB units. I would think it might cause file system corruption as well. I believe Seagate has updated their drivers to fix this problem, but for some sort of reason, from what I've seen, these problems occur for the most part on Mountain Lion. My own theory is that these companies HAD kernel extensions loaded into the system based on an old 32 bit vs. the new 64 bit model and they either don't load properly, they don't align properly, or they flat out don't work. I'm speculating here, so feel free to chime in. In any case, one of the things I observed is that all the drives that seem to have problems also have drivers installed. I have an old SmartDisk external 2.5" FireWire drive, a cheap USB case that I use with different drives for backups, and an old Maxtor one-touch FireWire. They're all, oddly enough, old, with the Maxtor being the newest in the group at 5 years old and I have no problems with Mountain Lion. If you're looking at buying an external drive to use with Mountain Lion, do a web search and make sure it's compatible and has all the appropriate updates.