SOLVED Why is Time Machine disk busy?

[bladerunner714]

As a part of the multi-faceted backup strategy for my mac Mini running macOS Big Sur, I have a large capacity, always on, desktop external HDD dedicated to automatic Time Machine backups which occur approximately every hour. Often when there is no backup in process, I can see (and hear) that the HDD is working away at something. My guess has been that macOS may be indexing the contents of the HDD which, after all, are subjected by Time Machine to some volatility. More as a matter of curiosity than concern, does anyone know (or have a better guess about) what my disk might be doing?

 

[Nurgle2024]

try starting holding the D key down and run diagnostics, oe 2 reset the PRAM. your time machine may have gotten into a loop. I had that once, where it was constantly going, but no progress..

also go to Activity Monitor and see how much it is hogging your computer.
regards, Sandy

 

[Nurgle2024]

also try running disk utility and select your time machine disk and repair permissions.

 

[Nurgle2024]

loops occasionaly, very occasionaly can happen when it is trying to back up a file that you may be working on or deleted since it put it on its interal back up lits. normaly it will just drop it and move on to the next. but if it is in the middl eof writing th efile to the time disk when you change it, occaionaly it will "loose it way".
regards, Sandy

 

[bladerunner714]

loops occasionaly, very occasionaly can happen when it is trying to back up a file that you may be working on or deleted since it put it on its interal back up lits. normaly it will just drop it and move on to the next. but if it is in the middl eof writing th efile to the time disk when you change it, occaionaly it will "loose it way".
regards, Sandy

Sandy - Thanks for the suggestions. I'm pretty sure my Time Machine isn't looping. It finishes quickly and the backups appear to be complete. The HDD "busy" behavior begins after the backup ends and then ends a while later. Activity Monitor doesn't show Time Machine running while the HDD "busy" behavior is observed. Everything appears to be in order and, if the HDD didn't make any sound, I'd never have raised the question.

 

[bladerunner714]

A little more poking around ... Same behavior obtains on all three of my macs. Activity Monitor shows Spotlight is the culprit. Spotlight is, after all, the application that does the indexing. So the Time Machine HDD must be busy building its Spotlight index.

 

[Larry Denenberg]

I'd like to reopen this question about a different point in time: Why is my disk active *after* being ejected?

I don't leave external backup disks plugged in; when it's time for backup I plug one in, click "Back Up Now", wait for completion, and eject/unplug the disk. I notice activity after backup is complete, but I'm willing to accept that Spotlight indexing is in progress. (I sorta wish I could disable this, but you can't exclude backup disks in the Spotlight privacy settings.)

But after I eject the disk (and before I unplug it) it *still* doesn't become quiescent, or at least not for a long time. Can it be that Spotlight is still indexing even an unmounted volume? My (admittedly ancient) mental model of a Unix machine doesn't permit this possibility. The Activity Monitor doesn't show Spotlight behavior during this time, and neither does "top" which us old folk still prefer. What is going on?