Slow parity initialization of RAID-5 array on HP Smart Array P411 controller

Well, it's a little odd. I don't see many cases of mixing HP Smart Array controllers and Dell servers. Either way, the parity initialization doesn't begin until I/O is started on the new logical drive. May I ask how you're monitoring this? Via the HP Array Configuration Utility webpage? Perhaps the HP ACU command-line tool? If you have the latter installed, can you provide the output of:

ctrl all show config detail

We'd like to see that output to see if there's a potential issue with one of your disks.

From the HP Smart Array manual:

Background RAID creation 
When you create a RAID 1, RAID 5, or RAID 6 logical drive, the Smart Array controller must build the 
logical drive within the array and initialize the parity before enabling certain advanced performance 
techniques. Parity initialization takes several hours to complete. The time it takes depends on the size of the 
logical drive and the load on the controller. The Smart Array controller creates the logical drive, initializing 
the parity whenever the controller is not busy. While the controller creates the logical drive, you can access 
the storage volume which has full fault tolerance. 

Also, check the firmware on the Smart Array P411 controller. Do you have a cache module installed with a battery or flash backup? If not, you'll have other performance problems over time.

The likely-hood of a non-recoverable error in this day and age is extremely high. Might I suggest either a raid 1 or 10. Especially if this is holding anything important.