I like where hw-tph is going with this, but is this a stock Slack kernel? If so, then it has support for just about every chipset you could scrounge. Heck, what's this machine? And the drive? And did you use a ghetto old 40-wire IDE cable on an ATA100 drive, they don't like that.
A good chunk of "dmesg" and all of "/sbin/lspci" should be enough to go on.
Also, there's the no-brainer, check to see if DMA is disabled from the board level in BIOS. The machine might have switched to "failsafe defaults" from being turned off ugly once, who knows... and the moron mode that some of these bios's hop to is no fun at all, rockin like its 1988.