TM does seem to sulk now and then... never had it fail to recognise the external HDD, but a few times it's decided to stop backing up- everything seems to happen normally, but a look in the system log shows that only a few hundred bytes were copied over

.
I only noticed when, after downloading about 6GB from iTunes, it clearly didn't run for long enough!
Switching backups off then on again usually cures it, but once I had to do a full reboot.
Giving serious thought to QRecall...