When I changed the setting of a container to store data externally the change caused each record's modified timestamp field to indicate the data had been changed. It had not! it is just where you are storing it on the server. This may sound trivial but in my case caused many iPads to be resent 650 MB of identical graphics over 3g. At the very least I should have been warned in the message!
no update of modification timestamp
Turn off modification update before making change then back on afterwards