Fresh install (12.1-RELEASE) won't boot

[Ronald F. Guilmette]

Got myself a new toy... a cheap HP T620 thin client off eBay.

Installed FreeBSD 12.1 release onto the SSD and all seemed to go well.

Rebooted after the install and the dang thing apparently got hung
trying to do a PXE boot.  (I have no idea why.  It didn't do this
when Windows was on the SSD.)  So I disabled PXE as a boot device
in the BIOS... probably the Wrong Way, but I'm not sure it matters.

Rebooted again and now all I get is:

   ERROR: No boot disk has been detected or the disk has failed.

I figured that maybe it had something to do with Secure boot, so I went
into the BIOS and checked.  Secure Boot is disabled already.

:-(

What did I do wrong?