Sadly since Android 4.4 KitKat applications are no longer able to write or delete files from the external SD storage. This has been confirmed by Google on his Android Developer site in the following link https://source.android.com/devices/storage/index.html.
Applications can write and delete files on external SD cards BUT only on their system defined folder so there is no WRITE access to other folders anymore, for example camera folder in the SD. This is crucial for the correct usage of SlidePick so in Android 4.4 KitKat you will be restricted to use only the internal memory of the device.
We hope in time Google correct this issue but seems rather a long shot because Android 5.0 Lollipop is already been released making very difficult to have this issue corrected for a older version. Anyway if this problem is fixed we will update SlidePick so Android 4.4 users will be able to use external SD storage.
In Android 5.0 Lollipop the permissions issues seem to be fixed so our development team is working on this matter at the moment to bring full functionality on the external SD storage.