DMA weirdness and problems

[Nadeem Bitar]

I'm experiencing some very strange behavior with my external usb hard
drive, memory stick reader and dvd reader/writer  and I think the
problem is related.

Transferring large files is very CPU intensive and slow. My machine is a
dual AMD64 with fast SATA disks so transferring large files to/from
external devices should not be this slow. The same external hard drive
plugged to my laptop, which is also running Ubuntu 5.10, does not have
any problem.

What makes it weird is that the behavior is not consistent but always
repeatable. I can sometimes transfer 500MB of data with not problem and
at a very fast speed but transferring 100MB after that would freeze the
machine for 10 minutes.  

I tried setting DMA on those devices but it fails with an invalid
argument message. 

sudo hdparm -d 1 /dev/sdd

/dev/sdd:
 setting using_dma to 1 (on)
 HDIO_SET_DMA failed: Invalid argument

Any help greatly appreciated!