We have a disk which is on its way out so I would like to swap it before
it dies. This disk is one of 3 which make up an NSS pool. Is the only
way to do this to backup the pool, delete it, replace the disk, recreate
the pool and then restore from backup?

Presumably if I just took the one disk out then all data from the entire
pool would be lost?

Can I get away with leaving the pool as is. Replacing the disk, adding
the new disk to the pool and then restoring from backup?