Micro SD card is not recognised

There are a few issues that can limit the unit's ability to read a MicroSD card.

1. First, have you made sure the card is compatible? Sometimes the card itself is a problem. Can happen. I use only x10 or faster units and have had no issues (At the Moment, I'm using a Samsung 64 gig card),
2. Is the card CLEAN? Dirty cards with fingerprints on the metallic surfaces can be problematic although not usually so. But it can happen and using a slightly-damp towel is a plus (right after the shower anyone?),
3. The orientation of the card is paramount when inserting. As you face the unit with the home button down and the loading port for the card on the left, make sure the logo (in this case, Samsung) is facing up and the largest squared off end is out, smaller side in, metallic side down.
4. A satisfying, almost imperceptible *click* when it's inserted and is actually SEATED. Beware this one as it's important. I use the ROUNDED end of a paperclip to gently finish the parts my fat fingers can't reach.

After that, one should format the card if it's not done already by the unit's auto-magic formatting.

Settings > Storage > Erase SD card, the usual warnings about everything on it prior to it's being erased going bye-bye, yadda, yadda, yadda.

That's the procedure, your mileage may vary...