My time machine backups go to an USB disc that is connected to my wifi router. For some reason I got regular problems with time machine. It tells me “Time Machine completed a verification of your backups. To improve reliability, Time Machine must create a new backup for you.” Doing so deletes all past backups. It also takes hours to complete.
Someone made a nice post on how to fix this situation. I haven’t tried it, my mac is already rebuilding the backup, but it will be something I will try next time this error pops up.