This is a limitation imposed by the tinyFAT library:
tinyFAT is a very compact SD card library, produces code that’s considerably smaller and consumes less RAM than the standard Arduino SD library. I’ve tried the Arduino SD library (you may have noticed that the earlier versions of the source code had a macro define to switch between tinyFAT and Arduino SD), but it consumes too much resources that I fear would eat into the other planned features.
Thanks Ray. What do you think about installing 1284p instead of 644 to have enough RAM (and more flash) for OpenSprinkler?
1284p has 16K RAM – 4 times more than what you have right now, and it is also pin-compatible.
BTW, I posted my mod on Github: https://github.com/tony-osp/OpenSprinkler-experiments/
Also I started small blog to capture my OpenSprinkler experiments: http://tony-osp.dreamwidth.org/