Plug n go
No idea what OTG means but I just plug my Galaxy S2 into the computer and root around with Window Explorer until I find the files I want. The phone appears like a USB stick with it's own drive letter.
I think that the behaviour when connected can be altered but as I recall mine defaulted to USB stick emulation.
Device MUST be unlocked, i.e. not at lock screen.
In my case:-
Photos are in \DCIM\Camera
Downloads in \download
bluetooth downloads in \bluetooth
screenshots in \Picture\screenshots
You clearly want to be very careful not to damage the system files. They may be protected I don't know and don't want to find out:-)