Should be possible. Is it a USB external drive? I don't think that Mac OS 9.x and earlier Drive setup utility can format drives in external drive containers (I could be wrong, but seem to recall this). You could format and partition this on Windows but it won't format HFS/HFS+

If you have a Mac that runs OS X you can repartition/reformat this, you can also resize (shrink) the existing partition and leave the remainder as free space (which you can then attach the external to your PC and format the reclaimed free space).

If I didn't have Mac OS X, I would use a G-Parted Live CD and boot up on the Windows PC with it & the external drive attached. If it detects it (and I'm pretty sure it would), then you can reformat/repartition the external to whatever you want (HFS/HFS+, FAT32, NTFS etc etc). You can even resize the existing Mac partition if you want to (if you have the space on the disk) and not lose any existing data, then reformat the remaining space as NTFS or whatever.

Make a back-up of any data on the external first, regardless of how you go about it.

It might be the Partition Map used by the drive.

The drive used on OS9 is almost certainly using APM (Apple Partition Map).
Windows will expect MBR (Master Boot Record)
Intel Macs use GUID / GPT (Globally Unique ID, I think).

From memory, OSX 10.4 and later on PowerPC can mount, read and write to GUID partitions, but can only boot from APM.
Intel Macs can only boot from GUID, and can mount, read and write to APM partitions.
Windows 7 will use MBR to boot - unless it's EFI rather than BIOS hardware - but can probably mount, read and write to GUID / GPT.

I don't know if OS9 can read / write GUID partitions.

Here's a simple link that explains it 100x better than I can:
http://macs.about.com/od/usingyourmac/qt/partition-types.htm

And here's something else I've just found, never tried it, but I will this weekend:
http://hints.macworld.com/article.php?story=20071026051259471

Are you thinking of sharing partition( between OS9 and Windows 7 ? You may be limited to DOS partitions then. You'll need the Joliet and other File Access extensions in OS9. If it's a large hard drive, you may need another Mac, with OSX 10.3 or later, to format the partition to FAT32, I don't think Windows 7 (or XP or the V word) will let you format it as FAT32, only NTFS, which would be a non-sarter for OS9.

It should be possible, I've used DOS FAT16 and FAT32-formatted PCMCIA cards in OS8.6 and later, just don't expect to be able to boot from it.