This is odd. So I edited the file as you suggested, and this time the patch command completed successfully. But when I restarted dropN900, there is no change - no 'purge' button is there. Restarting the phone didn't help (didn't think it would).
[EDIT] Never mind. Re-read the original post, it only works for photos in the DCIM folder. And it seems the 'Sync Media' button in dropn900 only applies to the DCIM folder on the internal memory, the external card is ignored.