In a virtual machine I created an ISW mirror pair RAID array using sdb and sdc. I then tried to use GParted from the Live CD to partition it with a GPT and create a file system.
When starting GParted I got the following dialog, twice:
==== Libparted Warning ====
/!\ Not all of the space available to /dev/sdb appears to be
used, you can fix the GPT to use all of the space (an
extra 9208 blocks) or continue with the current settings?
[ Fix ][ Ignore ]
Once for /dev/sdb and once for /dev/sdc, the two physical drives on which the ISW mirror RAID array was created.
It appears that libparted is finding the GPT on the underlying disks of the RAID array and is probably offering to move it. Allowing this to be fixed might well destroy the RAID array by moving the primary and/or backup GPT on those disks to the start and/or end of the disk, overwriting the metadata.
When trying to create a partition under GPT on the RAID array it failed at the point of creating the file system because the partition device /dev/mapper/isw_bbhghdaeaf_Volume01 didn't exist.
To report the status of the RAID array run these commands:
sudo dmraid -r
sudo dmrais -s
It appears that GParted is broken and not able partition firmware specified software RAID arrays. It might even be dangerous to use.