HP ACU shows parity initialization failed on Smart Array 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.

100% safe ? no. No operation involving an array rebuild is fully safe, in particular with RAID 5. Will it work ? Probably.

Your problem is that a single read failure during the rebuild will cause the whole volume to fail. And you're going to do it twice, including once with a drive that is already having trouble.

Ideally, in such a situation you should take the system out of production, make a full backup, delete the RAID volume, change your disks, re-create the array and restore your backup.

If you really can't take that much downtime on that array, then you should try to change each disk separately and wait each time for the rebuild to be done but do NOT do that without having a full backup first and, if you intend to keep that system active during the rebuild, make sure you warn the users first that it is possible that they will lose all data after the last backup date and that you got their approval (after all, it's their data and they should be the ones deciding what risk they'd rather take: safe but show with downtime or unsafe but potentially with uninterrupted service).