It has gone all wrong - trying to replace a disk in my volume

TrueNAS General
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8

That’s likely because the pool is not mounted to /mnt.
Would you care to share the outputs, formatted using the </> button of ?
zpool status
camcontrol devlist
smartctl -x /dev/ada# for all the relevant numbers #

Also indicate what’s your motherboard (how many SATA ports?), whether you’re using a HBA (or a “SATA card”), ECC RAM, etc.