Why can’t the HDD’s in tray 1 and 2 in the D5-300C be created RAID by other 3rd party software, but HDD’s in tray 3, 4 and 5 can?

The D5-300C has the RAID controller to control the RAID for HDD’s in tray 1 and 2. If these two HDD’s were created RAID by 3rd party software, the RAID controller would be incompatible with the one D5-300C already has. And as there is no RAID controller for the HDD’s in tray 3, 4 and 5, these three HDD’s are supported to use 3rd party software to create RAID.