I've been building Raspian nodes with external storage for awhile.  I've found the recent changes to the boot eeprom to work just fine to boot from USB. With Raspian.  With Ubuntu - not so much. At this point, I consider it a bit of a rabbit hole.   So I'm fine with booting initially from the MicroSD card.  It doesn't get mounted - and that is perhaps a good thing as I can still debug a boot problem separately from the state of the MSD device.

The downside is that after kernel rebuild - that kernel is not installed automatically.  So I'm a few revisions behind already.   Resolving this issue with automation is currently deferred.  Once I figure out a manual method that is reliable - I'll document that here after some testing.

Another note is the hardware used.  The PCI-e to USB bridge boards I'm using use the RTL9210B-CG chip on a generic adapter.   The good thing is: no heat issues. At all.  The problem?  The device has a firmware setting that turns off power ( IOERROR! ) after 10 minutes of no access.  The upside?  Microk8s hammers that disk regularly - so as long as Microk8s is running properly - the disk stays on.

I disable all that power stuff ( to no effect, apparently ) with:

 - Disable sleep    

sudo systemctl mask sleep.target suspend.target hibernate.target hybrid-sleep.target

  - Re-Enable sleep

sudo systemctl unmask sleep.target suspend.target hibernate.target hybrid-sleep.target