> mount_ffs: /dev/sd0a on /: specified device does not match mounted device

Root must be somewhere other than sd0a.  Late in boot there's a line
saying something like "root on sd0a dumps on sd0b".  What's the "root
on" device from that line?  If it says sd0a, your /dev is probably
corrupted, but more likely it says something else (sd0c, sd3a,
whatever), in which case that's what you need to try to mount on /.

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