Can't delete certain files/folders on SD card. (MM 6.0.1)

Stock, unrooted Marshmallow 6.0.1

I'm trying to delete some music files and folders on my SD card. I can use Solid Explorer to copy files over my LAN to my music folder. Once I copy, I can then delete those new files/folders off the SD.

The problem is with older files and folders. Solid Explorer and the built-in My Files app both fail to delete, with no information explaining why.

I'm guessing the problem is with any files and folders that were put on the SD over USB. 35GB would take too long over WIFI.

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