umass BBB problem

[Patrick Welche <prlw1%cam.ac.uk@localhost>]

I plugged a

umass0 at uhub1 port 5 configuration 1 interface 0
umass0: JMicron USB to ATA/ATAPI Bridge, rev 2.00/1.00, addr 2
umass0: using SCSI over Bulk-Only
scsibus0 at umass0: 2 targets, 1 lun per target
sd0 at scsibus0 target 0 lun 0: <SAMSUNG, HM160HC, 0-10> disk fixed
sd0: 149 GB, 16383 cyl, 16 head, 63 sec, 512 bytes/sect x 312581808 sectors

into a -current/amd64 box and cp -R 40GB of files with no problem. Plugged
it into a -current/i386 of same vintage (well, 8th April for i386,
14th April kernel for amd64) and keep getting

umass0: BBB bulk-in clear stall failed, TIMEOUT
umass0: BBB bulk-out clear stall failed, TIMEOUT
umass0: BBB reset failed, TIMEOUT

repeat when trying to copy the same files. In both cases the copy is from
the external disk to the computer.

How can I spot the difference?

Cheers,

Patrick