AMD64 system, "bios" type firmware, 2 SATA drives

Added 3rd SATA drive, now it boots from the 2nd drive rather than
the first.  I had to rearrange the power connections to add the 3rd drive,
so suspected power connector to 1st drive was not plugged in securely.
Fixed that, and unplugged new 3rd drive.  (The data cables were not moved.)
NetBSD boots from 2nd drive and sees 1st drive, and mounts partitions from
1st drive.

But I still can't boot from the 1st drive.  Both drives have NetBSD boot
selector in MBR.  Before adding the new drive, I could switch back and
forth between the drives via function keys F1 and F2.  Now I just
get "error".

I happen to have a copy of the 1st drive's MBR from before adding the 3rd
drive.  I read sector 0 with dd and they compare, so the MBR didn't get
corrupted.

I went into bios and had it "auto-detect" both drives, didn't help.
Did a full power cycle (incl 5V standby), didn't help.

Out of ideas, and I need to boot the 1st drive.  :-(

What am I missing?  Even if the bios is messed up so that it boots
the 2nd drive, why doesn't the NetBSD boot selector switch to
disk one when I press F1?