The key thing to remember with IDE configurations is that the computer can’t access two drives on the same channel simultaneously. Once a request is outstanding on one drive, the other drive on that channel is inaccessible. This is an architectural limitation of IDE compared to SCSI (and one reason why SATA has gone with a ‘one drive per channel’ approach - all SATA drives are masters).
There are apparently still some situations where certain drives don’t like to be slaves - but I’ve never come across one recently.
My personal rules for IDE configuration are that a hard disk has a channel to itself, and that optical drives can share. The qualification with optical drives sharing a channel is that you may hit problems trying to use both at once - so if a reader and a writer share, you may struggle to copy disc to disc between the two (though these days most programs go via the hard disk), and if the drives are two writers, you may struggle to use both simultaneously.
However, there’s no hard and fast rules - you may find that combinations that are theoretically sub-optimal for performance work fine in practice. For example, unless two hard disks are accessed heavily and, more importantly, simultaneously (as they would be in a RAID setup), you can probably get away with two hard disks sharing, though I would try to avoid it. After all, most hard disk accesses, particularly when streaming data, are over in a few milliseconds. Optical drive accesses, particularly when burning, can be rather longer, which can lead to problems if you are trying to access (and especially burn from) a hard disk on the same channel as the writer. Even then, such a combination may work.
Two hard disks involved in a RAID on the same channel could well be a performance hit. Are your storage pair really RAID 1 (mirroring)?
Anything else is really experimentation, and the best setup may depend on how you use your system. Personally wouldn’t want anything optical (and especially a writer) on the same channel as my boot drive, especially if it contains the Windows swap file.
I’m fortunate really - in my seven drive setup, two hard discs are U320 SCSI, the tape drive is U2W SCSI and the CD-RW is Ultra SCSI. That lot is across two SCSI controllers - the LSIlogic U320 SCSI controller on my motherboard controls the hard disks and tape drive.
The CD-RW is on an Adaptec 2920C Fast-10 SCSI PCI card with my scanner. Fast-10 is more than fast enough, and my 29160N U160 SCSI card doesn’t work properly under Windows XP - thanks a lot Adaptec - I have the choice of Microsoft drivers that tend to leave all narrow devices operating in asynchronous mode which makes the CD-RW unusable as a writer, or Adaptec drivers that hang Windows at logon if the scanner is attached (you usually finish up with a controller error in the System Event Log if Windows doesn’t crash utterly before it gets that far). I might look for an Ultra SCSI card like an Adaptec 2940AU on eBay.
That leaves my slowest hard disk (only used for backup to disc and as a scratch space) and two other optical drives as IDE. The hard disk is primary master, the two optical drives share the secondary channel.