Hi
If you stay under 4.5gb there should be no problem using an external drive even if it is a 68pin with an adapter.
An affordable solution that i have tried and found to work well is an Iomega Jazz drive. These come as 1GB and 2GB drive sizes, as they use removable discs like zips( but 10x more memory) the max size of your library is unlimited. You can often find the more common 1GB drives on ebay for between 10-25 UK pounds. I can recommend these as the discs are more easy to come by than the 2GB model.
My problem is that i wanted to use a larger drive and step it down to 50pin, but even though it should work in theory with the right adapter, finding a reliable one is proving to be dificult.
I have one large external 68pin drive that has the correct adapter and sits inside a centronics enclosure and this works great, however I have yet to build a working one myself. Every atempt I have tried has failed. I have used 4 different 68pin drives ranging in capacity and tried 2 adapters (including The one I bought from ebay paying 20 pounds UK because this one had High bit ternimation) but I still had no luck.
A converter WILL do the job as one of the drives has proved but it has got me stumped.
Anyway I can work with the 40gb of drives that I have, and as I have the external drives formated in FAT I am able to back them up on my PC freeing up space should need arise.
GOOD LUCK WITH THE MURKY WORLD OF SCSI