Open case with power-OFF and rseat the CD cables on both ends. Check results and see if works. If not, then the CD drive is hanging that buss and will cause the system to be slow or appear so, thus the ext. USB drive can hang. I would try another CD drive to help troubleshoot it, if possible. I found CD drives don't last long if used alot. You can try other IDE cables if available. I wouldn't emilinate the p[ossiblibity of a weak power supply as well. Though Dell, may offer a clue if you look at the LEDs on backside when problem occurs, but check after boot-up what they look like, then try accessing CD drive.
Dell PC has a CD-ROM that will not recognize and the following msg is given: "Windows successfully loaded the device driver for this hardware but cannot find the hardware device. (Code 41)".
When I utilize an external USB DVD-ROM, it will no longer recognize either and is given the same message.
Any help is appreciated.