booting from wrong disk

[PJ]

While trying to learn and understand the dump-retore process, I messed
up the ad4s1a partition and could not boot. To fix it I restored a
dumpfile of ad12s1a which is, for all intents and purposes, the same as
ad4s1a. I then boot from ad4 and surprise, surprise...
#df shows we have been booted from ad12 and all partitions are ad12....
Booting from ad12s1a gives exactly the same results.
So, how can I get ad4s1a to boot from ad4?
I imagine it is something in the boot files... but how to fix that?