Backup aborted - destination volume disappeared

For the second time in a row Carbon Copy Cloner could not finish backing up because "the backup volume disappeared".

What caused it this time was me going to a different desktop by swipe - this was the moment the external hard drive disappeared almost an hour into a large backup (because of moved FCP X libraries).

What is the cause of volumes disappearing?

I hope it's not a digital Freddy Kruger.

thanks!