I used Titanium mobile sdk 22.214.171.124 and KitchenSink to test filesystem api, but it seems Filesystem's write API does not work on device which can work on emulator.
On device, when i open a file by Titanium.Filesystem.getFile, it is always with the 'readonly' property to true and can not write data into file. I test with KitchenSink, it seems failed too.
On the iOs device the resourcesDirectory is readonly. Try to write to applicationDataDirectory