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Windows will not allow you to delete the pre-loaded media files which shipped along with the phone. But these files will not occupy much space. Try searching for files above 1 mb size using search option. Find and delete unwanted files which occupies more space and not important to you.

1. Make sure the storage card is properly inserted into your device.
2. Tap Start > Programs > File Explorer and navigate to the appropriate folder.
3. Tap and hold the file you want to copy, and tap Copy.
4. Tap the Down arrow () then tap My Device from the list.
5.Tap on My Documnets
8 Tap on My Pictures , select and delete the default pictures from here
9. Tap on Up on the lower left corner
10. Repeat step 8 for files under my videos
11. Do the same for My music , if you have any default music files



Thanks
Proton