Time Machine is not backing up
All of a sudden Time Machine is not backing up this Mac. I have let it run all day, and what I get is this under the pull-down menu:
Here is a screen shot of the Time Machine preference next to the external-hard-drive icon on the Desktop:
Time Machine reports that the current backup should be larger than the available 60.5GB of space. But Time Machine promises to delete the oldest backups to make room for the new.
I have been using the same external hard drive for four or five years, without incident. As we see, its icon appears normally on the Desktop. As an experiment, I attempted to run Disk Utility on the external hard drive. Disk Utility got as far as checking the catalog of the external hard drive. Then Disk Utility stopped operating. Disk Utility did not progress beyond checking the catalog. I let Disk Utility alone for more than an hour without any result. I tried to quit Disk Utility. All I got was am audible system beep. After two or three such attempts, I did a force-quit of Disk Utility and restarted the computer. Now I am back to where I was before: Time Machine is "preparing backup."
I opened the icon of Time Machine Backups. All my existing backups are in there.
Does anyone have any insight as to what the problem with Time Machine is?
Mac mini, macOS High Sierra (10.13), null