Time machine cant resolve destination problem fixed

I use an external USB 1Tb drive for my time machine backups. I back up maybe 750Gb of data on this. It works when I do it first but then after a couple of days, it says that it can't resolve the time machine destination and the time machine backup fails.

I went googling this morning and discovered that if this happens then you just need to select the time machine disk again in time machine options and then it starts working again.