[Edit 23.12.2011] CAUTION! People trying the below mentioned modification of the Promise Thunderbolt reported issues and data loss. Please consult the Promise Support if you are planning to do modifications or already did!
[UPDATE] with the announced Hitachi Deskstar 7K4000 4000GB(4TB), 5K4000 as well as the 4TB Seagate Barracuda XT a total of 24TB could be reached when modding the Promise Pegasus just replacing the 2TB drives it comes with. 5TB HDDs are reported arriving in January 2012. Read our updated Article: Caution with Promise Pegasus Thunderbolt RAID 6x3TB (18TB) or 6x4TB (24TB) modification. 6x5TB (30TB) possible as of January 2012?
The Promise Pegasus is the most “promising” hardware for enthusiasts and pro users, since it offers really fast transfer speeds, even topping the OCZ Vertex 3 SATA 6Gbps internal SSD on Macbook Pro computers.
Delivering over 800MB/s of disk performance, Pegasus is compatible with Mac systems with Thunderbolt
Compare the Promise Pegasus R4 (4x 2TB) and the Promise Pegasus R6 (8x 2TB). Of course, if you want to pimp your pegasus, you can replace the internal drives with different ones.
Internally Promise uses a PMC Sierra PM8011 8-port SAS-2 RAID controller. This is an 8-lane PCIe Gen 2 controller with eight SAS/SATA 6Gbps ports. On the R6 obviously only six of those ports are functional. The PM8011 has an embedded 600MHz MIPS processor and is paired with 512MB of DDR2-533. (via Anandtech)
Possible replacements and hacks for the Promise Pegasus:
* replace the internal hard drives with SSD drives: at the current date this still has stability issues. We are waiting for updated drivers or a response from Pegasus about the current freezes and issues with SSD drives. If you really have to test it out, make sure you get 6Gbps SSD drives. 2.5inch drives will also fit in the caddy.
* replacing the internal drives with 3TB drives: this is a confirmed and stable way of pimping your pegasus drives. Pegasus comes with 2TB Hitachi Deskstar 7K3000 drives, so make sure you are getting 3TB drives from the same brand. We can recommend the Hitachi 3TB 7200RPM drive, but also we can confirm the Pegasus runs with the Western Digital 3 TB SATA II Intellipower 64 MB Cache Bulk/OEM Desktop Hard Drive and the SEAGATE Barracuda XT 3 TB SATA 6.0 Gb-s 64 MB Cache. Just keep in mind, not to mix up brands: you should get 6 times the same hard disk for the most stable system.
* replacing the internal drives with SAS 6Gbps Hard Disks: while this would render the promise pegasus a real pro system that is rock stable, drives should be used, that offer a compatible SAS-connector. Since SAS-drives are generally compatible to SATA-connectors, we can generally recommend the Seagate Constellation ES.2 3 TB Internal Hard Drive SAS 600 7200rpm 64MB
Conclusio: The Promise Pegasus is a fast solution for great read/write speed plus the extra feature of having the possibility to daisy-chain more Pegasus devices together, to increase the space when needed. Compared to the Data Robotics DROBO PRO System, it only offers 1-drive fail (1 drive can get corrupted, if 2 fail your data is lost) while the DROBO offers dual drive redundancy. Also, the DROBO PRO supports different sized drives, while you can only put drives of the same size (and it is recommended to also only use the exact same drive models) in the Promise Pegasus.
Overall, nanofunk is recommending the Pegasus R4 and R6, because of its faster speed and possibilty to enhance the system. Since we had our DROBO PRO replaced twice (the unit itself was faulty) we are currently fully in favor of the Promise Pegasus.