I’ve noticed an annoying problem with OS X 10.7.2 (Lion) and Time Machine. Maybe you’ve seen it, and maybe not, but I thought I’d mention it here and how to work around it.
Sometimes I need to log out of my machine – occasionally, something will start acting a bit oddly, for example. I usually try logging out before rebooting as it’s much quicker. If logging out and back in doesn’t work, then I reboot.
But often when I log out, then log back in, my Mac will attempt to do a Time Machine backup – and fail. It claims the Time Machine disk is “read only” and can’t be written to. I’m not sure why it thinks that – maybe some sort of timing problem with the drive mounting but not yet being 100% ready. All I know is Time Machine is dead.
The fix? Turn OFF Time Machine (in System Preferences). Then go to Disk Utility and unmount your Time Machine partition. Remount the partition, wait for it to come ready, and then turn Time Machine back on. Tell Time Machine to backup now, and low and behold, the drive has magically changed from “read only” to writable again.