I have an old Mac Mini that I use to play around with some Mac apps, and to use as a Netflix machine. I noticed today that Time Machine was not finding the backup disk. This has happened several times over the last few months even though I never turn off the Mini, and never unplug the external drive Time Machine is backing up to.
Turning the drive off and on did nothing, unplugging it from the USB port and plugging it back in did nothing. I had to reboot the Mini to get Time Machine to find the drive again.
Will have to keep an eye on this.