Hello,
I have an external USB hard drive that, on one of my Slackware computers, I can only write to at around 2.3MB/s. It is USB 2.0, and I have checked this on another Slackware computer, and seems to work fine there. The hard drive is partitioned as reiserfs. I have checked the drive using reiserfsck and everything comes up alright there. Also, I will lose connection to the drive when writing to it on the problematic computer, where it stalls when writing, and then I have to physically disconnect and reconnect it to the computer. I am running Slackware-current on the computer that is experiencing this problem, and mostly a base 10.2 installation on the other one where there does not appear to be a problem, if these factors matter. Thanks.

Make sure the USB port is a 2.0 port, not a 1.1 port.

Another thing, if you're using a usb hub, then the speed will be limited to the maximum speed of the hub.

I'm saying this because I found the same thing in the USB flash drive I was using when I used it with and without the hub. I wondered why it was so slow until I realized that it was connected to the hub.

You can generally see the USB info using the KInfoCenter app (in KDE System menu if you're using KDE).