Extremely slow USB to USB transfer

[Jingles]

Hi IriDium, mk01,

Thank you so much for your replies.

The drives are both 1TB portable hard rives formatted with ext4. They are always connected to a powered USB hub as they are bus powered drives, and the Pi doesn't like it if I plug them into it directly. (@mk01 I can't test them when formatted with btrfs as they both have 400GB+ of movies and TV on them atm!)

I didn't get chance to look at this until yesterday. I ran the hdparm -tT on the drives connected in a variety of ways and got the same results each time – both drives scored 100+ MB/s on the cached read, and 'vexed' scored ~20 MB/s on the buffered, while 'vashet' only scored something around 1-300 kB/s. I don't think the drives were mounted during these tests. The connections i tested:
*vexed only on /dev/sda and /dev/sdb
*vashet only on /dev/sda and /dev/sdb
*vexed on /dev/sda and vashet on /dev/sdb
*vashet on /dev/sda and vexed on /dev/sdb

@IriDium I think that rules out it being /dev/sda that was causing the problem. And whether i have one or both drive connected, and the USB port. The powered hub is plugged into one port and the drives have been switched around on the hub itself many times.

Monday night I also rebooted my Pi as I was tidying the power cables for my media setup and ran some tests afterwards. 'vashet' now reads fine and it is 'vexed' that is reading slow! They are currently attached with vashet on /dev/sda, vexed on /dev/sdd (despite what the mount output says!). I have attached the mount, fdisk and hdparm commands for the drives as they are currently attached. I mentioned earlier that I have disable the auto-mount feature in xbian and use fstab to mount the drives now. The fstab entries are:
LABEL={label} /media/{name} ext4 defaults 0 0

I have also noticed that whenever running any of the commands above the Pi always take a while to get the data for the disk that is reading slow. For example if I run fdisk it spews out everything very quickly, but then takes maybe a minute or two to get the information for the 'slow-disk'. I have seen the outputs come out in the wrong order to (say the 'slow-disk' is on /dev/sda my fdsik output can read: dev/mmcblk0 info, then dev/sdb info, pause then dev/sda info. It is similarly slow to put the headers for the hdparm command on screen when running that on the 'slow-disk'

I have tried to be a clear and comprehensive as possible in my descriptions, but am still relatively new to Linux so if there is anything I have missed or need to clarify then please say. I look forward to hearing your suggestions!

Jingles