The purpose of DMA is to accelerate your harddisk access. But on some old harddisk, DMa is not supported, by enabling it will cause that harddisk fail to read and write properly.

By disabling DMA, doesn't meant you can't access your harddisk. Knoppix will acces it, but slower.

What I try to do in
1) is to disable DMA on that particular harddisk. Meaning, only unsupported harddisk will run slow (DMA disable), and the other harddisk can take advantage over DMA.

2) By appending nodma in your lilo, all harddisk will not be able to use DMA. Meaning, all harddisk including supported DMA also will run slower.
See the example how to add nodma in your lilo, and don't forget to run command: lilo in root konsole after you edited your lilo and saved it.