It's not working on the Dell 8400 while it works elsewhere. So let's not call it missing.
The Dell 8400 may have front USB ports. Many have the wrong wiring type. Workaround? Use the rear USB port.
Fixed?
Good.
Not? The list gets long at this point. From installing a .INF file from Intel, changing drive letters to TWEAKUI. I'm going to list them in the order I'd perform.
1. TWEAKUI. (google.com) Check that drive letters are not "hidden."
2. Change drive letter. In Disk Management assign the drive a new drive letter.
3. Install the Intel driver from this link. It's for the Intel 925X Express Chipset.
http://www.intel.com/support/chipsets/sb/CS-012398.htm
"Intel
I have a Maxtor 3200 external hard drive connected to my Dell 8400. All of a sudden it just stoped showing up under My Computer. It does show up in the divise manager and shows as working properly. When I hook it to my Dell laptop it works fine. Any help would be appreciated.

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