Could you try LIDO on your SE/30 and the LaCie box to set the interleave maybe?

LIDO 3.X used to be on Macintoshgarden, but the site is down (again).

I might look if I stored it locally, in case you would like to try.

I offered my old (SCSI) devices here for costs of shipping:

http://www.macdomain.org/index.php?topic=464.0

Such a Syquest beast might be suitable for a Plus.