Does anyone have any recommendations? 4 drives minimum, nothing ridiculous maximum. SATA/SAS I'm not hugely concerned as long as throughput is at least in the 500MB/sec-or-pretty-near class in RAID5.
A single drive averages 100MB/s (@ 5X00 RPM)- 140MB/s (@ 7200 RPM). In Raid5 with 4 drives you get (3x100MB/s) 300MB/s - (3x140MB/s) 420MB/s before controller overhead, thunderbolt overhead, etc.
2 possible solutions:
1 TB Easy: Buffalo MiniStation Thunderbolt Portable SSD
read speed up to 350MB/s great random reads compared to HDD, write speeds about half of read speeds. Maximum capacity is 1TB.
Multiple TB Complicated: Sonnet Echo Express SE II external PCIe anclosure with a Raid controller with external ports external enclosure with compatible ports. Same speed as internal solution, but comes in two parts. With Four drives in Raid5 you can't exceed the speeds listed above, with more drives higher speeds are possible.