Windows not recognising external hard disk/flash

I have a dell dimension 8300 with System Service Tag 350021J

My computer is not recognising any external hard drives or flash drives in any of the USB ports, although other devices in the ports (such as my mouse) work fine. I've checked my main external hard drive and flash drive in another computer and found that they are both working fine. As a result, I thought I'd try looking at the Dell drivers page for my computer. I have tried restarting the computer and installing the chipset download from the dell website, but neither of these solved the problem. There didn't seem to be anything else on the Dell website which may correct the error. At the moment I have two problems which appear in device manager - a question mark by "SCSI/RAID Host Controller" and another by "PCI Simple Communications Controller". These have both had question marks by them for a long time and have presented no problems before.

I have exhausted all the ideas I had for solving the problem and wonder whether there is anyone with more experience than me who is able to offer some advice regarding this problem.

Many thanks in advance