Yeah, specs say 4 SAS ports only through the external connector and 4 internal SATA ports. The external SAS port can be used for SAS or SATA, the internal SATA ports are SATA only.

The external connector assumes you're using external enclosures. If you want to you can loop it back inside the case but it'll require the case to have up to 8 SATA power connectors which is slightly unusual (assuming you're using the internal SATA connectors as well).

I'd have to read the documentation more for that board.

SAS drives can only connect to SAS controllers, SATA drives can connect to SATA or to SAS controllers but SAS drives cannot connect to SATA controllers.

If the 4 SATA ports goto a SAS controller then you could have 8 SAS drives connected at 6gbps or 4 connected "dual channel" at 12gbps or 8 SATA drives at 6gbps.

SAS is an extension on SATA. You can connect 4 SATA drives to most SAS ports or 2 12gbps SAS drives. You can't connect a SAS drive to a SATA port.

J12 needs a mini-SAS breakout cable and each drive requires a PSU. J5-8 are already SATA ports. 8 total SATA drives or 4 SAS + 4 SATA drives are supported by the ports given.

TechRadar: MDD - which is a white label brand owned by Goharddrive - sources stocks of what looks like new but EOL (end of line) hard disk drives.

Likely safe to use, but expect a lot of "not as advertised" for everything from speed to longevity.