I have a Plextor external USB based DVD drive which appears to perform to rated speeds when plugged into a 1.1 USB port. However, when we move it to a more contemporary system that has a USB 2.0 port, the performance degrades. Experiments with a USB 1.1 add-in card remedy this issue on the same system.

We are running Redhat Enterprise Linux.

Thanks in advance.

some motherboards will allow you to set usb version in the bios. worth a look.

checked with plextor for firmware updates?

USB 2.0 support is provided by the ehci-hcd module in linux. By removing that module, you can force all usb ports into USB 1.x mode which is provided by the uhci-hcd module. For USB 2.0 both the uhci and the ehci module have to be loaded; for USB 1.x, only the uhci module need be loaded. Accordingly, you can open a console, su to root and run the following every time you want to use the dvd drive:

# rmmod ehci-hcd

then if you want USB 2.0 after you are done with the drive, run:

# modprobe ehci-hcd

to reload the module. If you never want to use the USB 2.0, you can prevent the ehci module form loading by editing /etc/modprobe.conf or /etc/modules.conf depending on what kernel you are running. Or you can remove it by adding the rmmod ehci-hcd command to the end of /etc/rc.d/rc.local. Also, check your lsmod output for the exact name of the ehci and uhci modules your distro uses and adjust the above accordingly. IIRC in older kernels they were simply "ehci" and "uhci" while newer kernels use "ehci-hcd" and "uhci-hcd". you have the use the exact name when doing the rmmod and modprobe commands,