@Magren did you ever figure out what your problem was? I also use a RPI3B+ and the following set up:
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Trascend JetDrive 500: https://www.transcend-info.com/Products/No-624
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Apple MacBook Air 2011 original 120GB SSD drive: https://beetstech.com/blog/apple-proprietary-ssd-ultimate-guide-to-specs-and-upgrades#hdr-7
I’m about to pull the trigger and order a Crucial SSD drive but would rather prefer to use the one that I have lying around.
I imaged today an old 8GB memory stick as per @DavidFW1960 suggestion above, and the RPI did boot up. I could see lots of outputs displayed on a physically connected monitor.
To make matters a bit more weird, when I try to boot off the 120GB SSD drive (and nothing happens - not even a single line being output on the screen), not even a USB keybarod seems to be recognized because not even the CAPS LOCK led comes on when I press on it.
Any ideal?