SlackAlice must be a mindreader.
My next question was going to be about the SATA cables. I've only encountered cables with two ends on and am mulling on whether a split cable could be problematic.
My knowledge is a bit outdated but, at one time, master/slave was configured by either, or both, of (i) a jumper setting on the drive chassis or specially modified data cable and which socket went where.
I just hope this split SATA isn't fussy about what is plugged where.
Main thing though, is there any creasing or signs of folding in the cable? Tight spaces may make this unavoidable. Once or twice (at the factory or at maintainance/repair times) might be okay but repeated flexing will eventually crack the copper.
Have the mobo connectors sustained any damage?
Any dust in the, previously unused, connector?
(Reaching a bit, but...) Any screws, tools, wire fragments, dropped inside the case during disassembly?
I was going to mention dust in general but an 18 month machine should still be pretty clean, except for the heatsinks and fan vents. Dust + humidity = weakly conducting bridge between mobo component contacts.