The storage array's that say the capacity and speed of the drive on the caddy like 300GB, do you have to use those drives or can you put like 5TB drives in them even if they are older units?
Depends on the unit. Most aren't going to be SATA if they say 300GB, so generally no. Many SANs won't use regular drives anyway on account of using 520/528 byte sectors as opposed to 512. If you're looking at eBay and want a cheap NAS, build your own.