Hi.  The computer I am having problems with is  Dell Dimension 4700 from about 2004 with WINDOWS XP and 2 GB Ram, I don't really know the other specifications.  Well I have a few flash drives, 1 of them is a 1 GB Cruzer which sometimes works with it, one if a 4 GB cruzer that I haven't used on this computer, and one is a 16 GB OCZ Deisel which will NOT work on this computer. 

I have tried basically every USB port in the computer, and when I put it in you know that noise it usually makes in XP when there is a low pitched note and then a higher pitched note?  When I put this drive in I get 2 of the low pitched notes and it doesn't read the flash drive.  I don't know why it doesn't work, since it works on my Vista computer and all the XP computers at my school... maybe this computer is too old?? Any other answers? Thanks.Do you have any USB devices (printer, etc.) that work on that computer?It's also confusing because you mention 3 different units...
Apply the following questions to each flash drive:
Does it work on another machine ?
Are you using front USB ports or rear ?
Did it ever work ?
Was it purchased on eBay ?
Does it show up in Disk Management ?I actually found a solution to the problem.  the flash drive wasn't getting read by the computer, but I went to Add Hardware in the Control panel and it recognized that I had the flash drive in, it just wasn't doing ANYTHING, so I added the hardware and found the drivers for it, and it works now.