Help needed - recovering zfs pools after zio_read error: 5 ZFS: i/o error all block copies unavailable

Hi -
I hacve a FreeBSD 13 system. I got "zio_read error: 5 ZFS: i/o error all 
block copies unavailable " while attempting a reboot. I've verified that 
hardware is ok and that disks can be read. I booted the system off a 
flash drive. Prior to boot I loaded kernel and zfs.ko. After searching 
and trying a number  of things I finally found that "zpool import -f -R 
/tmp/mnt zroot" would actually mount what I assume is the root volume. 
It contains tmp, var, usr, and zroot.   However, there was a directory 
that apparently, while appearing in "/" was not actually a part of that 
pool. There is no sign of "/exportables" the missing directory.

I don't know where it is. ZFS was set up with all the defaults presented 
by the installer. Has anyone any idea where to go from here ?

The system has 2 drives morrored. So I thought about a fresh install on 
one drive and resilvering from the other but I'm not sure if that would 
work or how to go about it.

Ant ideas would be appreciated.

Thanks all,
Gene