I have a U3 card, serial #: SDSQXN-032G-C46A. The card reads and writes fine on my PC, I've checked for errors and authenticity with live support and H2Test.
The issue that arises is that it reads and writes fine on my phone as well, but it will not work properly when in recovery mode. Installing zips from the SD in TWRP will result in some sort of error 50% of the time. I am certain it is the card because I swapped my old Kingston Class 4 card in, and it worked perfectly; I've moved the zip to the internal SD, and it also installs fine.
I'm currently wondering if the S3 doesn't support UHS-1 U3 downgrading speeds to Class 10. I'm aware the S3's controller doesn't handle UHS-1, so it should downgrade to Class 10. Any solutions?