Chosen Solution
The SD-Card reader of my mid-2010 MacBook Pro 15.4” seems to have a loose contact. The SD-Card is sometimes read, sometimes it is not. I tried contact cleaner spray which seemed to help once, but didn’t fix it sustainably. Any other idea what I can try? (I’d rule out re-soldering the card reader on the logic board, I’d probably break more than fix)
Sadly the SD card reader is a soldered on connector to the logic board so the only solution is to replace the logic board to fix it. Do be careful on which SD cards you are expecting to work the newer SD versions are not supported in the older macOS’s Frankly, I would just use an external USB card reader and call it a day!
