Yosemite left a message on my desktop last night saying that one of my four internal hard drives on my Mac Pro was not ejected properly. I didn't eject it; the system did sometime in the middle of the night. Finder, of course, doesn't show the disk, but then neither does Disk Utility (yes, I restarted before looking). Is this the sign of a catastrophic failure done quietly? Is there another way to get a heartbeat from the missing disk? Thanks.