While not the
only problem, the one that bugs me far and away above all others...
This has been a widespread problem since Big Sur, affecting a wide variety of external enclosures that- through same cable, same firmware, same enclosure, same drives- have no such problem hooked to PRE-Big Sur Macs. Through an enormous amount of testing and trials, I've mostly concluded that there are bugs in macOS port management that only Apple can fix... and they haven't got to it through now 3 generations of macOS.
Remedy: try different enclosures until you find one that can remain consistently connected. The "Apple is always right" cheerleaders will jump into every thread about this problem and try to blame anything and everything except Apple but all that seems to fall away when one can connect through the
same cable to a pre-Big Sur Mac and all is stable again.
There's even posts of people who had stable enclosures pre-Big Sur, upgraded to Big Sur or newer, crash into the problem, and the stable connection being important enough to them to then downgrade back to the pre-Big Sur OS on the
SAME Mac. What happened when they did? All is fine again. That seems to scream source of this problem, as it rules out
every "redirection" variable.
The issue is posted by many here and all over the web, including Apple's own support forums. It is not limited to any one brand of enclosure, any particular age of enclosures, any particular Mac, only Silicon Macs, etc. It seems to be most problematic with RAID HDDs though single HDDs show it too. It seems least problematic with single SSDs though all are not immune.
There is a long list the cheerleaders will recommend to try to resolve the problem but, generally,
nothing works for most other than flipping back to pre-Big Sur or giving up on an enclosure and trying a different one. At some point, I expect Apple to update port management software and the U in USB will mean what it is supposed to mean again. For now- on our dear Macs- sub in an M for that U, to mean "MAYBE" or "MIGHT" (work reliably).
As in all things, 10 or 20 guys will pound in to say they have had no problem whatsoever with this (implying that those having troubles must be at fault, or conspiring against Apple, etc.) but there's just too many with the issue for it to be user error, bad cable, settings, third party stuff, firmware, etc. Some enclosures have no problems so some users do NOT experience this until they attach an enclosure that will unexpectedly eject.
My old iMac and my getting-old MB both running macOS BEFORE Big Sur have no trouble at all with a "mission critical" enclosure I like to use. But my "latest & greatest" Mac Studio Ultra running latest & greatest macOS can't maintain the same connection through the same cable for more than about 3 hours MAX. And I'm farrrrrrrr from the only one.
Apple, if you ever read threads like this, PLEASE make the U mean what it is supposed to mean again. We miss "just works" Apple. Could we get back to that?
The image at the top of this post is the result of yet another fresh test just before I posted this on an up-to-date Mac Studio Ultra: made it a whopping 37 minutes this time before unexpectedly ejecting. 37 minutes!!! 🎉🎉🎉