The updated Google Play Music application on Android finally lets you cache your music in the external storage (rather than the internal memory), and you can now play music stored in the external storage offline.
To change the storage location, do as follows:
1. Open Google Play Music (Make sure it is updated to the latest version)
2. Tap the 3dot icon and select Settings.
3. Under Downloading settings, open Storage Location.
4. Now, choose External as the storage location
All your saved music files in the internal memory, will be immediately moved to the external storage and the later music files will be downloaded and cached on the external storage too.
You may find the cached music files under this path:
** Note that, this method only works on Android devices running KitKat and some older devices like Galaxy S4