Raid1 Issue on RPI4

[Bruce Labitt]

So I got the raid1 array running on the RPI4.  Today, I tried to add a 
sub directory to the array.  Apparently something bad happened and one 
of the disks disappeared.  When this happened there was some kind of 
major upset, as the OS stopped functioning, like it no longer knew where 
commands were located.  sudo, stopped working.  I could type in commands 
via ssh and see the characters, but the command interpreter didn't 
function correctly.  I got a bash message saying it couldn't find the 
command.  Could not establish another ssh session with the RPI.  At that 
point, I pulled the power.

Upon a normal reboot, I find sdb is missing.  md0 is still intact with 
just sdc1.  I'm not sure how mkdir would cause this, but...

$ sudo mdadm --detaill /dev/md0 states there are 2 RAID devices, but 1 
total devices.  1 Active device, 1 working device, 0 failed devices, 0 
spare devices.  Disk0 state is removed, and Disk1 state is active sync 
/dev/sdc1

Assuming I can get sdb back, how do I get it back in the array? Just $ 
sudo mdadm --manage /dev/md0 --add /dev/sdb1 ?  Is there a way with UUID's?

Weird that the NVME disk sdb has just disappeared.  Even after a reboot 
it isn't present, even though the idiot light is on.  fdisk doesn't show 
it.  parted allows me to select it but print shows info on sda?  parted 
will select and print data on sdc.

Is there anything that can be done to 'rescue' the nvme disk sdb?