A few observations I made with my 64GB SD card.
When I first installed it, it didn't show up even when USB cabled to my PC. In checking I found that the mounting slot for the card doesn't always lock it in place. It is a bit flaky and you have to play with the card insertion until it doesn't spring back out a tiny bit.
Once I was sure the card was properly inserted it did show up on my PC, in the My Files app pre-installed on the phone, and Root Explorer App once I rooted the phone.
I simply connected the phone via USB cable to my Win7 OS PC and opened the file manager and dragged music and image files to the extSdCard and everything works just fine.
I used a new 64GB SD card that had not been in any other device so I don't know if you need to reformat in your case. I didn't have to format in my case.
I am sure there are real wizards on the forum that will give you more help shortly.