> Should i remove the failing drive from the mergerfs config and then rsync from the failing drive to the mergerfs mount point?
Sure. Or create a new, temporary pool with all the filesystems minus that one and rsync to that. That will allow it to happen while the filesystem in question is still accessible if that matters to you. Or attach a new filesystem, add it to the pool, and rsync directly to that. Really doesn't matter.
> Should i remove the failing drive from the mergerfs config and then rsync from the failing drive to the mergerfs mount point?
Sure. Or create a new, temporary pool with all the filesystems minus that one and rsync to that. That will allow it to happen while the filesystem in question is still accessible if that matters to you. Or attach a new filesystem, add it to the pool, and rsync directly to that. Really doesn't matter.