This is the message that I received on my iMac (late 2015) with one TB fusion drive. I also used another empty external 8TB hard drive withe the same result.
Right now, Time Machine is not creating new backups so the problem has to do with accessing the iMac hard drive??
iMac 27″, macOS 11.5
Lets try some basics first, I would recommend restarting the computer in Safe Mode with no wired peripherals (including the Time Machine drive) connected. Then restart normally, connect the Time Machine drive and test it.
Instructions for a Safe Mode start are in Use Safe Mode to isolate issues with your Mac
Welcome to the Apple Support Communities.
If it still doesn't work after booting into Safe Mode, go to Disk Utility, and select "View" > "Show All Devices". Select each device (or entry shown), and take a screenshot of it and upload them all here.
Resources:
Take a screenshot on your Mac - Apple Support
Cheers,
Jack
Time Machine could not back up the disk “Macintosh HD - Data” because a snapshot of the disk could not be created.
