I am unable to write files to my OTG usb storage
On lollipop, the 3rd party apps need to support the Storage Access Framework to write to external sdcards or OTG drives.  

The apps need to be updated to support Storage Access Framework, the app developers need to be informed.  

And considering the question old, many apps should have got updated by now. And the screenshot posted by you explains that the file manager is com.rhmsoft.fm which I'm sure is updated to support this feature as of now.  

And through modifying the platform.xml, used to work in kitkat, but is not guarenteed on lollipop. There are reports that it doesn't work on lollipop.