umass intermittently "stalling" ... processes seemingly stuck in "wdrain" state

I recently bought an USB Mass Storage unit (about 300GB) and a 4+1
slot PCI USB2.0 card (since it's an old box that didn't have USB 2 on
board yet), and i have noticed that intermittently the device "stalls"
any operations that try to access it:

- copying data onto the disk
- reading data from the disk
- running an ls while inside a directory on the disk's mount point

Today while trying to scp a fairly large set of files from my office
system to my umass disk at home i noticed the similar stalling
behaviour and decided to check ps output on my home system. There i
noticed that every single time to scp session would begin stalling
(lasting up to about 30-40s per stall) ... the scp process on my
home-system would have dropped into a wdrain state.

As soon as the scp process (after about 30 or more seconds) would
finally get out of this wdrain state (usually into an sbwait state)
the scp process on my work system would start picking up transfer
speed again until the next slip into wdrain.

I haven't been able to test this on a non-CURRENT system yet so i am
not sure wether or not this is a CURRENT only issue or a more generic
problem, but if somebody could give some pointers on where to start
looking on fixing this ... i am open for suggestions.

-- 
  Pascal