I'm wondering about that ESDI part, I tried an IDE drive on the bus and came up with absolutamente nada!
You'd know if you had an ESDI drive when you saw it, the cabling is completely different from IDE.
By any chance do you know what the brand/model number of the disk that's in it is? Also... did you try going into the BIOS setup, or did you just plug the new drive in and assume it would be autodetected? To digress quite a long way, well... you know the phrase "SCSI Voodoo"? When you're dealing with a machine as old as this you're going to encounter quite a bit of "IDE Voodoo". Amongst which:
A: Just to be certain, you have the drive you're trying hard-jumpered as "master", not "cable-select"?
B: The BIOS may not do autodetect, at least completely automatically. You'll probably have to drop into the BIOS and either:
1: Tell it to Autodetect (some BIOSes which *can* do it don't do it unless told), which may or may not work (* I'll get back to that), or if you're really unlucky:
2: Manually enter a C/H/S (cylinders/heads/sectors) sequence to describe the capacity of the drive you connected. I say "C/H/S" and not just an LBA capacity because:
C: This BIOS *will* have problems with drives larger than X, X being a number that could vary anywhere between 528MB and 8GB. A lot changed between about 1993 and 1998, a system produced between those years could have its personal limit stuck anywhere in that range; for a 486DX the most likely numbers are 528MB and 2.1GB. It's possible the BIOS in your system is so old it doesn't even know how to meaningfully detect a drive (IE, it doesn't know the "Geometry" command at all, and therefore drive setup is completely manual), or it could be running the command against whatever drive you plugged in and getting an answer that makes it wet its pants. (How large of a drive did you try?)
(Edit:
A history of BIOS drive size limits )
There are solutions for running big drives in systems like this; the most common is to use a "Disk Manager"; a common one was "EZ Drive", which used to be included with Maxtor and Western Digital drives on a floppy. They way they work is you set the BIOS up for the biggest size it supports (in your case, probably 528MB), and then use disk manager to install in the boot sector area a BIOS extension that gets loaded and "fixes" the problem (IE, it queries the disk's real size and sets up its substitute INT13 driver to use it) before booting an OS. Minus a disk manager your options are probably limited to using a drive smaller than the BIOS limit or using a larger drive and accepting that you won't be able to use part of it. (Note that the second case may only be an option if the larger-size drive doesn't crash the autodetection, if present. *If* it does then try disabling autodetect and manually punching in a size.)
Back in the day EZ Drive was great, and let you do all sorts of horrible off-the-radar things. I used it once, a copy that was provided with a new Maxtor drive, to set up a 486DX/33 with a really OLD bios to drive *three* castoff hard disks. All the disks were smaller than 528MB, but the old BIOS only supported a single IDE port. (IE, two drive maximum) The EZ Drive BIOS extension was smart enough to look for drives at secondary and tertiary ports, so I was able to hang the third HD off the IDE port on a Soundblaster card. The resulting Frankenstein machine even worked under OS/2. It looks like it's possible to download a copy of EZ Drive from Western Digital support, the one thing I'd worry about is if they since brain-damaged it so it only works with WD hard drives.
A system like this *would* probably be a good candidate for an IDE->Compactflash adapter, teamed up with a 2GB or smaller CF card. (Or heck, a 512MB card would be perfect.) Most CF cards still support C/H/S mode so it should be pretty much plug-and-play.