The app seemed to work OK and when I was done trying it, I disconnected the device in the program and closed the program, put the tablet into standby and pulled out the cable.
Seems the Tablet did not like this and locked up completely (blank screen). Had to "on-off button long press" to reboot it.
The LED on the Veho sits there occasionally blinking the tablet is in standby - even though the app is not running. So presumably that is not a good state to remove the device.
BTW I don't think this is a Nexus Photo Viewer issue - I repeated this after a reboot without running the app (although when plugging in the cable the app asked if I wanted to run it, I said no).
Anyway my question is - apart from powering down the Tablet completely, how else can I remove the OTG device without a Nexus 7 lockup?