Somehow I can't believe that the SCSI drives in those Powerbooks can't do better than ~540 Kbps. The 10MB hard drive shipped in the original IBM PC XT was capable of around 80KB/s (ie, 640Kbps) when paired with the original controller and formatted at a 6:1 interleave. (It improved to around 100KB/s if you reformatted as 5:1. Replace the Xebec controller with a good Western Digital that could do 3:1 and you could push 200KB/sec.)

If they really are that slow the SCSI subsystem/driver software must really, really, seriously suck.