sd card not mounted after formatting
Summing up the solution from above comments:

While it's not clear what happened with the card, the solution was quite trivial:


take the card out of the Android device, and put it into a card reader
attach the card reader to a computer
format the card using FAT32 (a file-system both sides can handle easily)
"safely remove" ("eject") the card from the computer, take it out of the card reader, and put it back to the Android device


Now the card was recognized fine \o/