I think jabooters is referring to connecting an internal drive to one of your IDE controllers. There may be two IDE controllers and each one can connect two IDE ATA/ATAPI drives.
To go back some ways it was suggested that you should partition and format the hard drive BEFORE installing it into the enclosure.
An unformatted drive in an external enclosure may not show in My Computer.
Are any of the drives you have tried already formatted. Keep in mind that Win98 can't read NTFS file system.
This thread has gone on far longer than necessary. Post all details of all the hardware you are using so that whatever the problem is, it can be identified.
Are you sure all the drives you have tried are working?
You are connecting both data and power connectors from the enclosure to the hard drive? Usually removing all jumpers from the drives works best. Some older drives might have jumpers to limit capacity installed on the back of the drive. Remove them if present.
The enclosure is being powered by 120VAC?
Power switch on enclosure is ON?