This took a long time. I mount the T5 less than once a day and I mount the T7 about once a week.
With both the T5 and T7, a common sequence is for the drive’s icon to disappear from the desktop, the drive’s red light to come on (often alternating with the bright blue light), the drive’s icon in Disk Utility to turn gray, and then the drive’s red light to turn off (sometimes with another cycle of blue-red before the red turns off). After the red light turns off, almost half the time, a faint blue light remains; just over half the time, the faint blue light goes off.
What does it all mean? Well, for me, it means I wait until either there is no light or the faint blue light, with no change for at least three seconds, before I disconnect the cable. If anyone is disconnecting the cable while the lights are blinking, I’d like to know if there have been any ill consequences (but I probably won’t change my behavior).
Related to the title of this thread, I finally installed macOS 11.6.5, with no apparent problems (yet).
That sounds like what I would do under the circumstances.
If someone disconnects the cable before the drive finishes whatever it is doing, I would expect the file system journal to prevent any data loss, but I would also expect it to take longer (maybe as much as a minute or two) to mount the next time it is used, as the system inspects the file system and replays the journal.
Since I wrote my post, I’ve been working with a WD Elements hard drive. It is formatted as ExFAT (for full read/write compatibility with Windows). When I eject it, the icon disappears immediately. Disk Utility shows it unmounted a second or two later. A few seconds after that, the single white LED goes into a slow-blink state (2 seconds on, 2 seconds off), which I believe means it is idle and safe to disconnect.
I’ve also changed by clone/backup (hard drive) volumes from bootable (four volumes in the APFS container, including bound system/data volumes) to data-only backups (only one volume in the container). It still takes a few seconds after the icon disappears until the unmount completes and a few more seconds until the drive’s in-use light stops blinking, but it is much faster than it was when there were two bound volumes trying to unmount together (which would usually take more than a minute).
Now I’ve found a problem. Similar to what I experienced with 11.6.1 (and reported in macOS Big Sur 11.6.1 and Security Update 2021-007 Catalina - #19 by Will_M), Time Machine is no longer completing backups, either on a Time Capsule or an external (spinning) drive. My recollection is that many days or weeks later, Time Machine just started working again, but maybe it was when I upgraded to 11.6.2.