Joined: 01 Sep 2013
Should be updated to new extSdCard access method.
I don't know the exact specifics, but Google seems to have implemented a new method of actually giving programs write access to SD cards. It seems an app can ask for permission which presents the user with a system allow or deny message box. Once granted the permission seems to stick if the user doesn't explicitly revoke it. I think it's one that says "storage" specifically. The old method of supporting legacy apps by rooted users with a writable system partition where one manually edits the platform.xml no longer seems to even work on newer versions of Android (I've done this and normal things that can write to the internal "sdcard" partition still can't write to the external actual SD card.)
I think specifically it must run through Android's own storage access system: http://android.stackexchange.com/questions/96500/how-to-allow-apps-to-delete-sd-card-content
Since 4.4 the "WRITE_EXTERNAL_STORAGE" permission no longer grants WRITE access to the secondary(external) sdcard. (This is a simplified explanation, 4.4 did not force this change.)
The necessary permission to do so ("WRITE_MEDIA_STORAGE") is only avialable to system apps.
Since 5.0 it is possible to gain write access through Androids storage provider system, this must be implemented by the developer though.
Expecting users to root and modify that system file looks like it will no longer solve the problem (if anything requiring root and modifying system files can really be considered a solution in the first place.)