endlessly 'preparing backup' in time machine

Since updating to High Sierra I can't make backups with time machine (spinning disk drive, Toshiba). In the Time Machine preferences it just says "preparing backup....." forever. And after about an hour, the attempt stops on its own.

But during this time I can hear the disk spinning loudly, and time machine and finder are using significant energy.

Here's what didn't work:

- Updating to the very latest version of OSX

- deleting the 'in.progress' backup file from the external hard drive, restarting and trying again.

Any new help would be tremendous.