Relocating an ext3 journal

[Pigeon <jah.pigeon@ukonline.co.uk>]

I have an ext3 filesystem on /dev/sdb1. I want to reconfigure it to
use a journal on /dev/sda1, as the SCSI drives in question are old
ones which are slow to seek, and I think it should improve performance.

So, I create an external journal on /dev/sda1 using

mke2fs -O has_journal /dev/sda1 -J size=100

and attempt to attach it to /dev/sdb1 using

tune2fs /dev/sdb1 -J device=/dev/sda1

which gives the error "The device already has a journal".

The information in man tune2fs (a) did not lead me to expect this
error and (b) doesn't say anything about how I might overcome it.

Can it be overcome? Or have I totally misunderstood this man page and
am within millimetres of nuking my filesystem?

Pigeon