Seems the previous thread has gotten corrupted, with only half my answer (the remainder is after the line break):
Hi Ivan,
Oddly, I had the exact opposite issue with various true IBM machines: PC330, PC350, PC750s - all around the same age as the machine he has. Also, if you look at the tech notes for older Packard Bells, HPs and CompuDynes, that is the configuration recommended as well.
It was somewhere in the slightly higher CPU (and BIOS) range that things started to change. The recommendations were for speed, do not connect to same channel as the HDD, and for compatibility/boot (and with Warp, sometimes even seeing it) set to slave.
Those are each the reason why the drives came factory set to slave (and the older machines seemed to prefer it - at least the ones that used the same chipsets and similar BIOS's to the IBMs) - because (1) it would be correct in a single cable setup, and (2) it would be correct and work in a dual IDE channel/cable setup.
The newer machines (the second series of 400MHz and up) preferred Master for CD-ROM on second controller for boot.
So perhaps I should have clarified that. His machine straddles that line (and cant tell on what side without knowing model information and such). Because it does leave a plethora of old machines that require it on the master setting for booting - but also a bunch of even older ones that use the slave channel.
I'm actually a little vague on the reasons - but I think it had to do with the ATAPI support on the drive and mobo... from the OS/2 CD ROM installation notes of that time (machines older than his, to about that age):
OS/2 Warp Installation With IDE CD-ROM
Symptom -
End user wants to know if OS/2 will work with IDE CD-ROM drives. Cause -
The cause of this issue is unknown at this time. Resolution -
OS/2 Warp and OS/2 Warp Connect will install correctly and work properly with all of our current model IDE CD-ROM (Panasonic 571/572, NEC 271/272) drives without software modification. This statement is true as long as the following two rules are followed.
1. The IDE CD-ROM is ATAPI v1.2 compliant.
2. The IDE CD-ROM cannot be configured as the Master.
This issue seemed to affect other operations as well (CD boot) on older machines with older ATAPI support, as well as a large amount of other drives - many at that time were made by those two companies (for instance, Creative's and IBM's drives were usually the Panasonics - and either labelled Creative, Creative-Panasonic, or IBM (with Panasonic's part number or for the multimedia kits, with Creative's part number (and when for IBM, Creative's part number with a letter appeneded and blue buttons).
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I think his kinda straddles that line (though maybe it is clearly "on the newer and this doesnt apply side") - but it has been ages...