Mac Pro :: RAID5 / 6 Rebuilt Times?

I recently bought the Areca 1880ix-16 card and the backplane cable to hook to the mac pro internal drives - hooked up 4 x seagate ST32000644NS 2TB enterprise drives for an internal RAID5 with 6TB of space. Used the DX4 from transinternational to hook up my 2 SSD drives. The initial raid build time was just about 2 days (!) for the raid 5. Shortly after that I left town for a few days and came back to an alarm in my computer - one of the disks had dropped out. The rebuild time was again close to 2 days!. At thipoint I realized that if I was sitting there with that much data at risk for a 2 day rebuild, it would be agonizing and so I needed to go raid6 (which had been my goal from the start, but only 4 internal disks is very limiting). So I gave up on managing my storage internally and bought an istorage pro it8SAS external box connected by miniSAS cable. The raid5 had to complete the rebuild before it would let me expand to raid6. I added another seagate ST32000644NS 2TB enterprise drive (interestingly the drive that was originally listed as failed has caused no further problems at all) and expanded the raid to RAID6 with the same capacity but one extra drive for parity. The rebuild now has been going on for almost 48 hours and it's at 53% I had NO idea the rebuild times would be this long. I do have 2.3TB of data on the array, so I'm not sure if that prolongs it vs. an empty array. But just for others info, take into consideration these times when deciding what your own needs are - I bought a high end (AFAIK) raid card, running in a high end computer (mac pro 3ghz octocore with 16GB RAM) and quality enterprise drives, and it's still maddeningly slow to rebuild. Hopefully once it's up and running, this will all be forgotten, and if I ever have a drive fail, I know I can still lose another one without any data loss. RAID6 seems the only way to go given these times for rebuild. FWIW, I also put in another 3 drives - old 500gb drives I had sitting around and built a secondary RAID5 array since I had the space, that completed in around 36 hours or so, and was done during the original raid5 rebuild.