Starting with firmware 2.1.5, we no longer support hosting the UI files on the SD card. We really never recommended it but now we have removed the ability for the reasons outlined below:
First, the SD card is very slow for the Arduino (leading to browser timeouts) and was very noticeable to the end user so for this alone the SD card hosting the files was never acceptable (in my opinion).
Second, the SD card files access from the Arduino perspective was a fair amount of code so by removing it we were able to add other features (such as weather adjustment options which OS 2.1 would not have enough space for otherwise).
Third, the OpenSprinkler does not require the UI files to function, at all. So by hosting them on the SD card you are not protecting yourself in any measurable way. Really what you need is the OpenSprinkler app. Every copy of the app has the UI files integrated and will never require an Internet connection. This means if you are using the Google Chrome extension, Firefox extension, OS X app, iOS app, Android app, Windows Phone app, Amazon Fire app, BlackBerry 10 app, or even opening index.htm locally on your computer, you won’t require an Internet connection.
For a full list of OpenSprinkler activities that require an Internet connection please refer to my list here: https://opensprinkler.com/forums/topic/os-regular-connection-to-the-cloud/#post-38210. This also highlights how the device will operate in the absence of an Internet connection.