I picked up an internal hard drive enclosure the other day from CompUSA that allows me to install an internal drive into a rack and slide it into my machine. This way, the drive is set up on my normal IDE chain on the motherboard, and (provided the machine is turned off) I can swap drives by simply replacing the drive in the rack. It works flawlessly on Windows XP, but on Red Hat Enterprise 3, it absolutely refuses to load during boot (I'm just using the drive for external storage . . . the linux installation is on one of the permanently mounted drives.) During the boot sequence, I repeatedly get the error message:

Modprobe: can't locate module ide-disk

The system boots ok, but the drive isn't available. All the other IDE disks on the system are found correctly, just not the one in the enclosure. The drive that's in the enclosure is a 300GB Maxtor (yes, my motherboard supports a hard disk that large, and Windows sees it just fine at the proper size.)

Any thoughts would be most appreciated.

Your probably getting the ide-disk error on boot because ide-disk is compiled into the kernel and the system is, for some reason, trying to load ide-disk as a module.
As for detecting the new drive, run as root:

# fdisk -l

and see if there is an entry for the new drive. This should list all partitions that the system recognizes, whether mounted or not.

I had already thought of the module problem and disabled the checking for the IDE module. Now I no longer get the error message, but the hard drive still doesn't get recognized within Linux using fdisk -l . . . which is very strange, because the bios clearly sees the hard drive on the system, and the drive works perfectly in Windows XP (perhaps I should take that as a note, I've been finding linux to be more and more difficult to troubleshoot and XP has been running great on my system.) I like linux and I like tinkering with it, but . . .