Does time machine need a "clean drive", or can I use one that has stuff on

You can, but it's going to fill up very quickly.
Also, you won't be able to back up the video files that are already on it.

Do you (or anybody else) know if you need to partition such a drive? Or can Time Machine use a part of a second hard drive without it being partitioned?

No need to partition as TM uses a folder with subfolders in it.
Just open System Preferences > Time Machine and see the available drives you can choose for TM backups...

Great thanks - I just bought a new 250GB 2nd internal HD for my PM G5, will use the new drive as primary and keep the older one for TM plus some few other folders including large X-Plane scenery. Thanks again.

When I installed Leopard it said that in order to use Time Machine it had to format my external drive. I have a bunch of files on there that I don't want to delete, and I don't want to have them on my computer's internal hard drive (that's why I moved them to the external), so I haven't yet enabled Time Machine. Where can I see the Time Machine folder and its subfolders?

Okay, I wish I'd seen this review earlier. According to this mid-year MacWorld review, the 'entire' volume must be dedicated to Time Machine. Now I'm back to wondering whether partitioning the drive into two volumes will work. I have a 160GB internal drive that I want to use for TM, but I'd still like to use a third of it or so for these X-Plane scenery files that I don't want clogging my main drive.

Here's the article: http://www.macworld.com/2006/08/firstlooks/leotimemac/index.php

p.s. I don't have Leopard yet, it's on order and should be here sometime next week.

That's an old article. TM put's everything into a folder called Backups.backupdb, and you can have other files stored on the drive without affecting them.

Do you know what format the drive is formatted as?

I partitioned mine. Media on one and Time Machine for the other partition. I just like it better that way. It’s a bit more simple.

From all accounts, the drive must be formatted as HFS extended (HFS+). Therefore, one would not be able to boot Windows off a Time Machine-dedicated drive.

"Do you know what format the drive is formatted as?"

Clicking on the drive and selecting "Get Info" tells me that the drive is MS-DOS(FAT32).

Thats your problem, TM requires the drive be formatted as HFS extended (HFS+). If the drive was already formatted as HFS+ then you would not have to reformat.

So is it possible for me to move all the files from my external back on to my computer, reformat the drive, then move everything back, and then enable TM?

Ok, Just for clarity....(getting ready for Leopard/Time Machine:
I have a 300 Gig Fire Wire WD that is formatted Fat32 and I have a backup of my MBPro data about 20 Gig.

* Can I do a a dual format... (dual partitions FAT32 (80GB) and HFS (220 GB) ? Or
Should I bite the bullet and format the WHOLE external drive as HFS and (move all 20 Gig Off the drive, format and move them back)?

I have a PC network and this is my only Mac.
* Is it true that anything I back up to HFS is only readable by a Mac?
OR
Perhaps there is a PC utility that lets PC's (win XP) read an HFS drive.
This is worst case where I might need to get at certain data, docs, pics...IF there is a MBP problem.

Thanks! -Bob O