Mike,  The Hot Spare provisioning recommendation is 1:30..  That is one hot spare for every 30 workload drives.  The reliablity of the 2TB drives is actually pretty high, and you won't be running them 7/24.  You'll also be in RAID 6, which has you double failure protected.  You can get by with a single hot spare, instead of two.
Hello community,  I am going to set up a CX4-240 as a exclusive backup to disk solution in order to back up the data stored on our NS4-480.  Hardware is:  2 DAE filled with 15x 2TB SATA drive 1 DAE filled with 8x 2TB SATA drive  So the total drive count is 38.  searching the forum for some hints, I found that I have to read the docs below and did it.  EMC CLARiiON Storage System Fundamentals for Performance and Availability EMC CLARiiON Best Practices for Fibre Channel Storage  After reading I considered to be on the safe side to use RAID6 in order to get double drive failure protection while the performance should still be good enough for backup to disk usage (sequential writes, large I/O and a steady I/O flow).  Unfortunately even after reading thoroughly I don't feel safe to decide on a RAID group size. I thought about 3 RAID 6 Groups (10+2) using up the first 36 drives and to have 2 Spares.  I'd be happy if anyone would comment on my assumptions and point out if I missunderstood something. Furthermore any recommendation on how to place the groups within the DAEs would be very wellcome.  Thanks in advantage and kind regards Mike   
RAID groups for backup to disk on CX4-240