I am planning on buying a NUC9i9QNX and have been looking around for information on installing a 2.5 inch SATA SSD (I already have a 4TB one I would like to use) but it seems that Intel has incomplete docs on the subject. Searching I found the following Intel doc:
On page 15 lists the NUC9i9QNX but when you click the link, you are taken to this error page:
I do not believe that there are any allowances for SATA HDDs in the chassis. There are discussions in the UG and IG regarding installation of M.2 SSDs but nothing about 2.5" SATA drives. There are M.2 connectors for SSDs on boards (with SATA lanes) and backplane (without SATA lanes), so difficult to support SATA drives in chassis.
There are no standard SATA connectors or power on the Element board or the back-plane so the only way to connect a 2.5" SATA drive is externally through USB. There are (3) M.2 slots in the kit so there should be plenty of storage available. M.2 drives are cheap now so no need to use 2.5" unless you have old drives you want to use.
There is actually a ribbon cable connector for a combined SATA/Power adapter on the board. Good luck finding this special connector and cable. Intel is not providing this cable for purchase. It is in the top right of the NUC Element board below the power connector.