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.
Just open my files < local storage < sd card. It contains all sd card storage items. You can easily delete from there.
You can delete the music files and videos in the music player. Or, you can also delete it by using the usb cable with your desktop. But i recommend delete the files in sd card through music player.