I am using a very vanilla macOS 11.6.4 Big Sur, which was upgraded from El Capitan originally installed on the iMac 27" retina (late 2015) machine. I am a developer, but for windows and Linux, not MacOS, so there's nothing of that sort installed or jimmied with on this machine.
Time machine used to work fine. It's going to a G-drive 4TB USB3 disk.
I somewhat recently upgraded from Catalina to Big Sur. I don't recall when my Time machine stopped working, but the last backup was Dec 6 2021. I can see the save set directory on the G-Drive corresponding to that date.
My first stab at debugging was to look around here for info about logs. I found a bit of advice given for dumping log lines on the console using this command line:
log show --info --debug --predicate 'process == "backupd"'
What I see is interesting; Time machine seems to be running, and deciding not to do anything:
<snip earlier...>
2022-02-23 06:15:28.406732-0800 0x6688c Info 0x0 297 0 backupd: (TimeMachine) [com.apple.TimeMachine:LocalThinning] Starting age based thinning of Time Machine local snapshots on disk '/System/Volumes/Data'
2022-02-23 06:15:28.407098-0800 0x6688c Info 0x0 297 0 backupd: (TimeMachine) [com.apple.TimeMachine:LocalThinning] Skipping age based thinning of Time Machine reference snapshot 'com.apple.TimeMachine.2021-12-06-090237.local' on disk '/System/Volumes/Data'
2022-02-23 07:16:03.211798-0800 0x6a714 Info 0x0 297 0 backupd: (TimeMachine) [com.apple.TimeMachine:LocalThinning] Starting age based thinning of Time Machine local snapshots on disk '/System/Volumes/Data'
2022-02-23 07:16:03.212160-0800 0x6a714 Info 0x0 297 0 backupd: (TimeMachine) [com.apple.TimeMachine:LocalThinning] Skipping age based thinning of Time Machine reference snapshot 'com.apple.TimeMachine.2021-12-06-090237.local' on disk '/System/Volumes/Data'
2022-02-23 08:16:08.356538-0800 0x6e385 Info 0x0 297 0 backupd: (TimeMachine) [com.apple.TimeMachine:LocalThinning] Starting age based thinning of Time Machine local snapshots on disk '/System/Volumes/Data'
2022-02-23 08:16:08.356905-0800 0x6e385 Info 0x0 297 0 backupd: (TimeMachine) [com.apple.TimeMachine:LocalThinning] Skipping age based thinning of Time Machine reference snapshot 'com.apple.TimeMachine.2021-12-06-090237.local' on disk '/System/Volumes/Data'
2022-02-23 09:16:54.522539-0800 0x74efd Info 0x0 297 0 backupd: (TimeMachine) [com.apple.TimeMachine:LocalThinning] Starting age based thinning of Time Machine local snapshots on disk '/System/Volumes/Data'
2022-02-23 09:16:54.935922-0800 0x74efd Info 0x0 297 0 backupd: (TimeMachine) [com.apple.TimeMachine:LocalThinning] Skipping age based thinning of Time Machine reference snapshot 'com.apple.TimeMachine.2021-12-06-090237.local' on disk '/System/Volumes/Data'
The other thing which looks strange about this is the lack of anything "off rhythm" of the once-per-hour scan which I would have expected to see around the 8:00 to 9:00 time interval, when I clicked "Backup Now" in the TimeMachine notification area menu.
Can someone give me a hint as to where I might dig around next? This is quite puzzling. (As I said, I've done a lot of systems-level development on Windows, and more recently with Ubuntu 18 as an embedded system OS on a network analyzer "appliance". If you point the way I can usually find my way around).
Many thanks, JoGusto Santa Cruz CA