You know, this actually sounds like the microSD card might be loose or not slotted into the cage correctly; have you done a visual inspection on it? The fact that Windows is reporting it sometimes there, sometimes not is a clue -- it smells like it's pointing to something hardware related that's wrong (SD card failing, USB port, etc.)
Have you tried a different USB cable as well? Any chance that cable could have gotten bent and the internal wires might be iffy?
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