If a single drive fails in a RAID, two blank drives of the same make and model will be sent as replacement for reliability reasons. Palo Alto Networks has determined that there are long term reliability issues with running RAID with mixed drive models, even if those drives are from the same vendor. It is not recommended to configure a system in RAID mode with different SSD drive models.
For more information on replacing drives and migrating data in a RAID configuration, please review the attachment PA-5000_series_SSD_Upgrade_Options.pdf