Try testing the external drive on another PC if you can to see if this
slow data rate is seen - no point delving into the OS / hardware if
it's the new drive that's at fault !

If the drive is a slim-line one ( i.e. uses a 2.5" laptop drive ) then
sometimes the USB port cannot supply enough power and it will default
to a slower speed unless extra is provided - often using a USB cable
with twin plugs on one end. The second plug goes into another USB port
and only has the power rails connected. This usually affects laptops
but I've seen desktops with front USB ports that do this.

Failing that, have you any other USB devices plugged in ? Depending on
how the ports are wired a USB 1.1 device can bring all ports down to
USB 1.1 - controllers cannot provide mixed speeds to connected ports.

Running lsusb in the terminal will tell you whether the system thinks
your ports are 1.1 or 2.0 and which one it's connected your drive to

Linux reckons everything's USB1.1. Some time in the next few days
I'll try a card I know for sure is USB2, and see what it thinks. Hopefully
that'll narrow down what's going on.

Are you plugging the device into the back slots? Or into some usb slot on
one of your peripherals?

Also, what the model and brand of your USB HD?

Its plugged into the back slots, no other usb device is connected, but there was
a very non-compliant usb cable in another slot. The USB HDD is a new usb2
Seagate 1TB ST310005EXD101-RK

At the moment the thing's still doing work. Once its done, which will take days,
I'll add a USB2 card and try it in that. Then I'll know more.

Can you explain more about "very non-compliant usb cable" and whether
this has been unplugged.

Also if the job is running that slowly you may get it done quicker
overall by stopping it / fitting that USB 2.0 card and restarting.