Ok, This is driving me nuts!
I have a 200GB Western Digital IDE hard drive. Originally I had it mounted on a ix86 Slackware machine. For kicks and giggles I tried putting the hard drive on my ix86 NetBSD machine (incidentally I use it as my router). At any rate, when I put the hard drive back onto my slackware machine it would cause the boot process to hang up at

Partition check:
hda: hda1 hda2
hdb:

and nothing else (I have let it sit overnight)

I brought the disk back to my BDS machine and noticed that it didn't have a disklabel any more (if it had one at all). so I used what the BSD kernel thought the label should be to write one back onto the disk. That didn't fix it.

At this point I'm willing to loose the data on the disk, I just want it usable on my linux machine.

Does linux use disklabels just as BSD does? or does linux use something else to mark partitions? I have been unable to find any mention of them in the linux man pages

is there any way to "Kick" the boot process to at least get the machine to boot the the hard drive attached?

And does anyone know what precisely caused the problem in the first place?

Take a linux rescue cdrom boot then do a chroot to your's linux partition and see what the problem is.See the partitions information with fdisk do a fsck or reinstall kernel ...
I don't know what cause the problem.
Why you do not reinstall everything if you are not interested in preserving data ?
Good Luck!

Maby I didn't make this clear.......
When the hard drive it pluged into the machine it wont finish booting. it stops at

Partition check:
hda: hda1 hda2
hdb:

I have let it sit there overnight and nothing happens.

My curent solution, that I havent gotten to yet, is put it back on my BSD machine and start playing with it from there.

It wont even boot with the Slackware install CD. it has the same problem

With the Slack boot cd you can boot from CD or from your's linux partition.
I've told you to boot with a kernel from your's slack CD and then try to use fdisk
to see if it is a problem .
If all is ok you can try to do a fsck on your's linux partitions.
If it is ok with the partitions to try to mount your's linux partition and after that if all is ok to do a chroot to the directory where you have mount the linux partition.
If all is ok you can try to view the /etc/fstab file.
If it is ok then try to reinstall kernel and probably a bootloader such as lilo or grub.
Good Luck !