As you are using MSA1050 which means you are using Virtual Disk Group (VDG).
VDG is not possible to expand in Virtual Array.
Vdisk expansion works in Linear array only. Till MSA2040 it works.
Only option is to re-create new VDG and add the same to the existing Pool. Otherwise delete the VDG you want to expand and then re-create it with desired number of drives.
Hope this helps!
Regards
Subhajit
I am an HPE employee
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