LordingMike, I understood that you want to set an ATA password for 2 SSDs, however hard driver password is limited to only one drive. That is the reason why this feature is not available when you choose RAID SATA mode.
The hard drive has to be connected to the SATA 0 port. This will enable the 'HDD Password' options on the BIOS.
In regards to the maximal length I would like to tell you that the password needs to be at least 2 characters and at most 8 characters long.