Have you tried Formatting USB key and put files back on It ?

Is it formatted in NTFS or FAT32 ?

Are the files on the root of the key or are they in folders , if so are the folder trees good :

eg: My Files / folder 1/ folder 2 / picture.jpg

Have you tried re making your folder trees and putting files back on it via a PC ?

The reason I say this is there was another user having issues with one of the newer VR range routers where the folder trees were messy and by re formatting
and rebuilding the trees it fixed the issue .

Tried each possible method
first with empty usb key formatted in FAT32 and NTFS.
Second with some media files on the root of the key without any folders and
third with files on the root of the key and some folder with media files without sub folder.

but still the problem is not solved. usb led doesn't stop flashing when router turned on with usb key connected.