why_wolf :
need to know what motherboard but usually its the first one or two SATA slots.
I don't know about "usually" since I've seen the last two (SATA4,5 starting with SATA0) mentioned several times too.
*Regardless, as said it is the MOTHERBOARD MANUAL that dictates and if you misplaced it just download a copy for your EXACT model of motherboard.
If there is more than one M.2 slot make sure you confirm the correct one.
(Also, if you plug your SATA drive into a slot and it is detected you know it's not that one that is disabled.)