I was able to source 3TB and 4TB drives from eBay fairly inexpensively; the 4TB Hitachi drives run around $200-225. Currently, the 3TB drives are few in number and high in price, but I was able to get mine for $115.
One thing I haven't tried yet is to see if the RAID array will automatically upgrade the storage limit when all of the drives have been upgraded. My Seagate BlackArmor NAS 440 will do that. Replace each drive with a larger size and when all are replaced the RAID5 array will embiggen itself.
The drives appear to be limited by the whitelist.xml file located on the thumbdrive image, and transferred to the System partition. Modifying that may allow the use of other drives. I haven't tried that yet, either.