By ISDN card, do you mean IDE card? ISDN is different and not really a hard disc interface.
I'm guessing you disconnected the CD drive to hook up the 3rd hard disc. The BIOS may well have "CD drive" recorded for that position. Typically, you can auto-detect on every boot, or specify the type of device. Someone may have set it to CD drive instead of autodetect. Also note that, if it's actually an add-on IDE controller card, it may have its own BIOS and configuration utility, you may have to watch during boot for a message telling you where to adjust ITS settings.
Also, if the drive is alone, it should definitely be set to "master" or "single drive" if available. "Slave" requires another drive hooked up, jumpered as "master" to come even close to working right.
IDE cables generally aren't twisted. Hard discs today work best when using cables rated for DMA/66 operation. You can tell them because, although their plug has 40 pins, you can see 80 wires on the cable. The cables are usually very stiff and have blue plugs, but sometimes they aren't.
I think twisted IDE cables are sometimes used with "cable select" operation (you jumper each drive to "cable select" setting, and hope they can negotiate based on their position on the cables), so that's also something to consider.
In order, I'd try:
-Trying to set the BIOS (or the card's BIOS) to "auto-detect" for all positons. Combine this with:
-Jumpering the hard disc for master
-Trying the "cable-select" jumper position
-Changing the cable for a standard one with no twist. Get a DMA/133 rated one.
If you only have four total IDE devices, why do you need an IDE card? Most recent mainboards give you two IDE sockets on board; the only reason you'd need the card was if you had a board far older than the drive, and it didn't support the latest modes. Usually, though, the latest modes are puffery; nothing saturates even the older DMA/100 modes and only the fastest drives in best cases fill DMA/66