I recently noticed one of my internal SATA drives wasn't mounted, and wasn't showing up in DiskUtility to let me mount it manually. I thought nothing of it until I started rebooting and STILL couldn't get the computer to see it. I've tried the drive in all 4 of my internal bays and looked to see if shows up under BootCamp but no luck yet. My other internal drives work in any bay slot, so I don't think it's a fault with the MacPro. As far as I know, it's spinning up and isn't making any dead-drive sounds I've heard in the past, it just seems to be invisible to the computer! This is the first time I've had a drive potentially fail and I -haven't- been able to recover it with normal recovery tools, mainly because, well, if it doesn't show up in the bus, how am I meant to recover it anyway? Does anyone have any advice for dealing with a drive when it's acting up like this?