I'm using a Nokia Lumia 520, with a 4 GB SD card.
How can I delete music/video files and folders from the SD card?
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You should be able to delete files from SD card trough Windows installed computer. Connect the phone via USB cable to computer and Windows should install basic drivers that will let you explore the contents of your phone.
If you can't then try inserting your Mini SD card into SD card adapter and then into card reader. By doing that, you will get full control & speed over your memory card. That's how I am doing it.