This sounds more like the backup and restore is being poorly explained. Even on a PC you can't use a file explorer to look at the backup area.
Since Android is the new Windows, you find a lack of documentation to be all too common.
Idea. format the card again but don't use sandisk's backup app. Find a replacement that works for you (meets your requirements.)
I just bought a 64 scandisk micro sd for my galaxy s4 and it is not working proporly I think. I downloaded the scandisk app that was recommened and there was a backup and restore function. I backed up my photos, videos ane music.
After like 1 or 2 hours, it finished, but I couldn't find it in the file explorer. It said that 25gb of 64 gb was used in the settings but I couldn't see it in the file explorer. I formated it and did the same many times, but the problem was still there.
if I bought an original 64gb samsung micro sd card, will all these these problems go away? Are samsung making scandisk not compatible so that you pay 5€ more for a samsung card? Or are scandisk cards just rubbish?
I have sent my scandisk back and ordered th3 samsung card