To make a VERY long story short, the issue was the SATA cable. The computer would boot from a conventional HD so I thouhgt the SATA cable was OK. Apparently, when these cables are failing, they can work at lower speeds, but then again may not work at all when trying to negotiate link speeds with a 6G SSD. I also noticed that with the bad cable, the negotiated link speed with the convnetional drive was 1.5 Gbps which was lower than expected. I replaced the cable from a reputable supplier and I have another SSD working. I think the SanDisk that I ended up returning was actually fine AND compatible.
I was clued in after a brand that I know for sure was compatible would not work either. Powerbookmedic sells relaible cables. “cheap” replacements are not recommended.
Hope this helps someone else.
