To regularly run tools like DiskWarrior, I have an external hard drive that will make my machine boot in root user, 10.6which is a fine facility to have. But now my MacBook Pro 15" (unibody, Mid-2009) won't boot up anymore off my external drive. For full disclosure, I should now add that I have not one but two of such external drivesand both don't work, while they still boot my 2007 MacBook and Intel iMac just fine. It makes no difference whether I try to use ALT during startup, or adjust the Startup Disk once booted up from the internal drive, in System Preferences. It also makes no difference whether I use the drives' FireWire or USB ports. So, what now? Time for a new motherboard, or... Anyone have a suggestion?