I also have the original Promise Pegasus R6 (6 x 2TB drives). I bought it when it was launched, and it has given me seven years of flawless service. Like you, I recently looked into upgrading the drives, but I was disappointed to discover that Promise no longer supports these units. It seems that they only support their products for about 3 years after release, and thereafter advise customers to upgrade to their latest offering. I can't afford to do that. Looking at their list of supported drives for the R6 (version 1), the 4TB Seagate BarraCudas are still available on Amazon, so I think I will buy six of them. I would prefer to use higher capacity drives, and I suspect they would work, but I don't feel comfortable with the risk. Six 4TB drives will give me 20TB of fast storage, and this will last me for several years.