booting from wrong disk

[PJ]

PJ wrote:
> 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?
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>   
add another thought...

If I change the mbr on the ad12 then mount ad4s1a to /mnt copy
/mnt/boot/boot0 to /boot/boot0.tmp and then copy the modified
/boot/boot0 (for ad4) back to /mnt/boot/ and then umount ad4s1a --- I
should be ok, OK? Have I got it?