You are going to need as many sd cards as you want, you will also need a reader for each one.
If you are going to use it with a mini, the you will need a usb 2.0 powered hub that has the amount of ports that you need. Now if you are going to put in a tower, I suggeest a usb 2.0 Pci card with at least four ports. If you are going to have more than four sd readers, then you can add hubs to that. This sounds like a neat idea, but it also sound ungodly expensive. Why not just raid some 1.8" hds? That would be small too and be faster. Make sure to post a mod guide if I didnt say that before. Good luck and have fun with it.