I just purchased a external hard drive and have it setup with this system. I plan on formatting the internal drive so I can upgrade to Slack 12. I should be able to access this drive after I do that right?

If by access you mean read it, yes you'll be able to access it after installing Slack. If you mean boot from it, well that depends on your system BIOS.

No, I dont need to boot from it, just access the stuff I have backed up onto it.
I bought it so I could backup stuff that would not fit on a CD in order to upgrade to Slack 12.

You should be able to see the external drive, but if you are worried try booting from a Live-CD version of Linux like Knoppix to confirm if you can see/use it properly!

That's good advice.

Here's the skinny: every "hard drive" is, hardware-wise, nothing more and nothing less than a spinning magnetic platter full of "sectors." To turn that into "files" and "directories" requires a file system, of which there are several choices both in Windows and in Linux/Unix.

For example, in the Windows world, there was originally FAT, which went through many incarnations as hard-drives got bigger and bigger. That filesystem never offered much in the way of security (and still doesn't...), so Microsoft replaced it with NTFS.

Meanwhile, on the Unix/Linux front, we have (as usual...) many native contenders, including ext2, its close cousin ext3, and other choices like reiserFS. All of these are ways, albeit different ways, to organize "a spinning platter full of sectors" into "files and directories."

Plop a CD-ROM or DVD-ROM into your machine and guess what? Another file-system, specifically designed to enable your disc to be understood on many different types of computers or devices. (And once again, several choices...)

Furthermore, every modern operating-system worth its salt .. certainly including Windows .. supports the notion of "installable" or "pluggable" file-system drivers. There's a well-defined mechanism, in all of these systems, whereby the operating-system can take a peek at a new device and, by querying the various installable file-systems, try to figure out which one is able to understand "this" disk. And in most cases you get precisely what you want: you "mount" the disk and in a split-second you can use the files on it.

When you're dealing with a removable device, the manufacturer of that device obviously has been obliged to guess what sort of system you're likely to be attaching it to. He probably decided "Windows," and he probably also decided, "old Windows," that being the safest choice because it won't cause his customer-support switchboard to light-up late at night. He probably chose FAT32: universally understood, but no security features.

Bottom line is... you probably want to re-format the drive: to install a blank file-system of your choosing.

That choice should be determined by ... which system(s) will need to be able to easily "understand" what's on this drive? Here it can become a bit complicated, because at this point I can no longer with-confidence say exactly what choices you have, nor how technically experienced you might or might not be...

Well, ive already formatted it with reiserfs and copied data to it, same as my internal hard drive.
As long as I use reiserfs with the new install of Slack 12 I should be able to access the drive right?

Should work no problems. I have three usb external hard disks ans slackware recognizes them and there FAT formatted. ReifersFS will be okay. If your stuck the best place to go is the Slackware forum...

http://www.linuxquestions.org/questions/slackware-14/

You maynot be able to access the drive right after installation due to HAL, there are a couple of things needed to perform from the command line to enable reading from cdrom/dvd's and ext hdd's, If I remember correctly. Check the above forum.