There is a bug in 10.8.5 that causes drives to spin-down or eject completely when the display goes to sleep. Even worse, the drives routinely will not properly wake up, requiring a full system reboot or at the very least, ejecting all external disks and remounting them.
To be clear, I have my Mac set to never sleep, and have the “Spin Down Drives” option disabled. For a couple of weeks, I was coming into my office, waking up my Mac’s display, and finding that Time Machine failed, and all four of my drives were reporting -50 errors when trying to write anything to the drive.
At first I thought my enclosure was going bad, or one of my drives. After some research, I found that many people are experiencing this issue with 10.8.5, and my issue did seem to start right after updating as well.
The temporary solution is set display sleep to Never. I just kick the brightness down when I walk away. Was having these hard drive issues constantly, but now haven’t had any problems in over 24 hours.
Apple is working on a supplemental 10.8.5 update to fix this issue and others with HDMI, Bluetooth and some other stuff. I’m looking forward to the update, but am just glad to have a solution in the meantime.