I struggled to choose a forum, because I wondered if this problem was caused by Office for Mac 2011?
I had several applications open and the computer was multi-tasking: items being copied across discs, video being edited, then (I'm guessing) Outlook polled my mailboxes in the background.
A dialog box appeared saying that Outlook suffered an I/O error and had to shut down. I relauched, but the app failed to launch.
Investigation showed that the Microsoft User Data folder was completely empty. I keep the folder on a drive dedicated to data, rather than OS/apps. A quick look showed that the Movies folder was also empty. 125 Gb of files had disappeared in an instant. A check of some file/folder names showed that they only appeared on my backup drive (thanks be...) WhatSize showed no invisible files or bizarre moves. (BTW, this Seagate drive is only one year old.)
I had a new drive on the shelf for such an emergency. I pulled DATA_B1 and put the new drive in its place, Bay 1. I thought I'd run DiskWarrior on the "failed" drive, so I plugged it into a USB dock. (I could not run DW or Disk Utility on DATA_B1 because it would not unmount. Disk Utility said it needed repair.)
It mounted and a quick look showed that all the previously missing files were all present and correct! I ran DW, ran Disk Utility, then launched Outlook using the original database. Worked fine.
I wondered if it was a SATA failure of some sort (I'm out of my depth here) so I put the Yosemite drive (which I rarely use) from Bay 4 into Bay 1 and moved everything down one bay. PRAM start, checked all the applications, confirmed backups were in order and everything is running fine right now.
Any guesses what it was I experienced?
The brand new Hitachi drive is back on the shelf and I feel chuffed that my years of computer training (backup/backup/backup) have paid off.