Extremely slow USB to USB transfer

[Jingles]

I think I have the cause of the problem but don't know how to fix it.

I have two 1TB external USB HDDs attached via a USB hub. I have some tests using dd to see the read and write speeds of my drives. Results show write speed of both drives is fine (~20-28MB/s depending on block size and amount of data) but the read speed for one drive is really slow which will obviously be causing the slow data transfer between the two disks. However when I attach the drive to a different PC it reads at 35 MB/s?! I have summarised the results below and also attached detailed copies of the dd results, and hdpram test, and fdisk outputs.

To me is seems there is a problem with the drive working on my Raspberry Pi but I have no idea what to do now to fix it. It is really an issue because the drive concerned has all my tv shows on and consequently I can't watch anything because xbmc can't get the video quick enough to play it!

I really hope someone can help

Drive 'vexed' read speeds:
19.1 MB/s (bs = 1048576, count = 1024)
122 MB/s (bs = 8k, count = 10k)

Drive 'vashet' read speeds:
446kB/s (bs = 1048576, count = 1024)
319 kB/s (bs = 8k, count = 10k)

When I attached the problem drive to another PC (desktop PC running opensuse 12.x) I got the following read/write speeds (bs = 1048576, count = 1024):
read = 35.8 Mb/s
write = 29.7 MB/s