macbook mid 2010 13" won't recognise ssd crucial mx200 HD 500gb placed in the optical drive
I suggest trying to put the SuperDrive back, and then seeing if the SATA cable for that works, if it does, you can switch the HDD for the SSD to check if it's faulty.

Also - there are inexpensive power supply / USB data connection kits so you could get one of those and test the bracket you purchased external to either MacBook and see if everything works over USB first.


the inexpensive self powered ones are fine in most cases
if you're wanting something to be sure you have a good part, consider getting one with a power supply. (or three since they are so cheap and as you know - sometimes a new part could be bad so having spares to test helps greatly). https://www.amazon.com/SANOXY-A12940-Drive-Adapter-Converter/dp/B001OORMVQ/


I wouldn't worry about a link negotiation between 3 Gbps or 6 Gbps - either will result in things working and even in the most stringent benchmark - it's almost never you are seeing more than 3 Gbps on a sata link so in practice you see no benefit from that - certainly not a cause for a drive not showing up entirely.