I currently have Slackware 10.1 installed on an IDE harddrive connected to my PC. For a few days however, I need it connected to my laptop. Therefor I was thinking I could buy a USB->IDE adapter and use that. I do realize that this won't just work out-of-the-box, which is why I need your help. How can I accomplish this?

Are you attempting to boot slackware (pc harddrive) on your laptop? Or are you just pulling data off of the drive?

I would prefer to be able to boot it. I could use a live-cd, but not unless I have to.

Be sure that your BIOS can support booting from the USB device. Some can, some cannot. If you don't see your USB device listed as an option as one of the boot choices, then realistically you would not be able to boot from it.

Not to go on a different tangent, but depending on the size of your laptop's hard drive, you could probably set it up as a dual boot Windows/Slack system. There are frequent threads about setting up dual boot systems here at LQ and a quick review would give you a good idea of what's involved. Good luck with it

I know the BIOS can boot from USB. The problem is (afaik) to get linux to mount the root fs when it's suddenly on /dev/sda1 instead of /dev/hda4. And similar problems.

I could of course set up a dualboot, but this is only for a few days, so it feels pretty overkill.