I had to add 2 hard disks to 4 extisting ones of a RAID5 which contained sensitive data.
First thing to say, it definately works on the fly without losing data (the drive doesn’t get formatted) – but to be sure, backup the whole content.
Then, open Dell Server Administrator and configure the drives with the task under “virtual hard drives”. It may take several days to finish the extension, but you can access the data in this time.
Note: Using more than 2 TB of the RAID is not possible.