Having worked for EMC for a number of years, I might be able to shed some light on this.
First, EMC always releases new drives with mirrored protection to protect the customer's data against any potential problems with the drive manufacturing process. Usually after about 3-6 months, depending on the field information), EMC will allow RAID-S. This was certainly true with disk sizes up to 36GB.
With the 72 and now 181GB drives there is a different problem: rebuild time. If you lose a 72 or 181GB drive it might take several days to rebuild the RAID-S group (based on activity). Unless you are using a dynamic spare, you are exposed to losing another drive in the same RAID group. It could be that EMC does not want to expose the customer to such a risk.
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This was first published in June 2002