I don't want to come across as a smart ass, but I'm not sure what you mean by newer external drives not working with OSX.
A blank hard drive doesn't care what the OS is. (For example, I have HDs from 2004 in a eSATA RAID array as one of my non-mission critical backup drives for my 2008 MacPro running Snow Leopard).
Once you connect a blank external HD it to your mini, it'll simply initialize it for your current OS.
What year is the mini? Even the first gens (PowerMac1.42ghz) have FireWire ports, etc., so there are many external drive options available, like a quad interface (FW400, FW800, USB, eSATA)' that'll future proof you.
http://eshop.macsales.com/shop/firewire/
http://eshop.macsales.com/item/Newer%20Technology/MSQ7S10TB32/
I get all of my externals from the above shop, as well as RAM and SSDs. Great company (I'm not affiliated, just a fan).