When starting the installation, add the string "hda=noprobe" (minus the quotes) to the boot command. For instance, if you do not have to add further parameters, you would boot the Slackware installer with this commandline instead of just pressing <ENTER>:
This will cause your disk to be recognized as sda again and enable DMA as well. Did you notice how slow disk access was when the disk was recognized as hda? That is because DMA is turned off and there is no way to turn it back on.
At the end of the installation when you get to the LILO configuration dialogs, you will see the prompt to add additional parameters:
Some systems might require extra parameters to be passed to the kernel.
If you needed to pass parameters to the kernel when you booted the
Slackware bootdisk, you'll probably want to enter the same ones here.
Most systems won't require any extra parameters.
If you don't need any, just hit ENTER to continue
Here, you have to enter again
so that your system will assign sda to your harddisk when it boots.
This issue is covered in the CHANGES_AND_HINTS.TXT file too, please read the documentation that comes with Slackware! We do not write these for fun.