Old MacBook Air 3rd party SSD visibility is temperature dependent 1st gen MacBook Air.. I replaced the failing internal HD with an 3rd party SSD (wasn't hard, but replacing the fan which was also broken was much trickier - pro tip: don't try using a 00 screwdriver on 000 screws). [will update this question with SSD make+model asap]

Problem: SSD visible on boot only sometimes, and oddly enough, only when the Mac is cold from being carried around outside (where its below 0 C right now). Also, when the Mac is initially cold enough to make SSD visible, it is usable, and can even booted from, even as the Mac is subjected to higher temperatures, until shutdown or sleep.

A little more history: When first connecting drive, didn't seem to work, but after several disconnections &amp; reconnections during process of replacing the fan (took a few tries since at first I first didn't have correct screwdriver, see pro tip above), SSD suddenly appeared on boot. I used it for about a day, after which I had to disconnect &amp; reconnect again (to finish installing fan since I finally got the correct screwdriver) which made it disappear again. I tried reseating connections several times, then finally gave up for the time being &amp; closed up the case, resigned to use my external HD exclusively. Only a few weeks later did the temperature outside drop allowing me to notice its temperature-dependant, semi-working state.

Question: has anyone ever seen such behaviour? Is it likely to really just be a matter of reopening case and reseating connections even though my repeated attempts at this have failed? (doing it wrong maybe?) Or might there be something trickier I should try?
If the original drive works inside the MacBook in any temp, then id say faulty SSD.

If not, then faulty MacBook :(

temperature based faults are usually categorised as things like dry joints, where solder between two surfaces is not correctly made and the contraction due to cold brings the surfaces in touch and heat creates expansion breaking the connection.