Just in case you came into the same problem than me.
Yesterday I rebooted my wnr and somehow the USB storage (USB stick) went read only. Unable to copy anything on it. Annoying.
In that case connect to the device and just do :
mount -o remount,rw /mnt
And that's it the share in now read/write.
I'm still kind of new to this, but from my understanding for some firmware you need s shutdown script which unmounts your drive before rebooting. I think this is caused by rebooting without safely unmounting your drive. I'm using Kongs newest firmware which has this built in, but it maybe something you want to check up on.