I have been frustrated by this issue for some time. I entered the command as suggested and got this response:
$ log show --predicate 'subsystem == "com.apple.TimeMachine"' --info --last 4h | grep Error
2018-01-05 10:56:14.895151-0800 0x4af425 Error 0x0 19858 14 backupd: (TimeMachine) [com.apple.TimeMachine:TMLogError] Couldn't get volume info for unmounted disk 'disk0s3'
2018-01-05 11:58:02.983719-0800 0x4b7b30 Error 0x0 19980 14 backupd: (TimeMachine) [com.apple.TimeMachine:TMLogError] Couldn't get volume info for unmounted disk 'disk0s3'
2018-01-05 12:57:55.972214-0800 0x4bcf32 Error 0x0 20085 14 backupd: (TimeMachine) [com.apple.TimeMachine:TMLogError] Couldn't get volume info for unmounted disk 'disk0s3'
2018-01-05 13:58:36.835014-0800 0x4c221c Error 0x0 20182 14 backupd: (TimeMachine) [com.apple.TimeMachine:TMLogError] Couldn't get volume info for unmounted disk 'disk0s3'
2018-01-05 14:29:26.372678-0800 0x4c9377 Error 0x128b5a 20542 14 backupd: (TimeMachine) [com.apple.TimeMachine:TMLogError] Couldn't get volume info for unmounted disk 'disk0s3'
This is quite different from your response so I think I have a different problem. Disk Utility didn't list 'disk0s3' (because it's unmounted?) and Spotlight didn't find it either. Do you have another suggestion to resolve this error? I sure hope so. Gerry