Prior to Android 3.0, Android allowed all applications to add/edit/delete content on the secondary storage. e.g. a camera app could create an image, a photo editor tweak it, and a tagger tag it.  However, as of Android 3.0 Google introduced restrictions that prevent 3rd party applications from writing to the secondary storage location such as an SD card. Until Android 4.3, most device manufacturers did not implement this restriction (though some devices such as the Motorola Xoom running Android 3.0 and some Sony Xperia devices running Android 4.0/4.1 did experience the issue), but in Android 4.4, this restriction was implemented on most devices. On Android 5 / 6, the restriction was again removed.

On devices implementing this restriction, attempts to sync/edit/delete content on the Primary storage (internal memory) will work as expected, but content saved to an SD Card or to an external USB drive may not sync/edit/delete as expected, and the following situations may arise:

1) Limited access to the secondary storage (for devices running Android 4.4+)

In such cases, MediaMonkey for Android will indicate that "MediaMonkey has limited access to this storage due to Android restrictions", and will automatically sync content to an Application-specific folder on the SD Card (i.e. it will modify the sync mask to sync to /Android/data/com.ventismedia.android.mediamonkey/...).  It's possible to sync, download, or edit media in this folder, the only limitations being that:

o Content stored to that folder can be read but not written to by other applications on the device

o Content in that folder is deleted when MediaMonkey is uninstalled

Note, however, that any content synced to the device before it was updated to Android 4.4, was probably saved to a shared directory on the SD card (e.g. SD Card/Music), and MediaMonkey for Android won't be able to edit or delete it. To resolve this problem you can either:

a) Use the File Manager that came with your device to delete the the content from the directory and then resync, and the newly-synced tracks will be copied to the app-specific directory.

b) Update the sync settings to move files that have already been synced to an application-specific directory.

o In the Sync Profile > Options > File locations > enable 'Enforce use of sync mask for files already on the device'.

o Then initiate a USB sync operation (not Wi-Fi sync) to move the tracks to the new location.

This will cause files that are stored in shared locations on the SD Card (e.g. SD Card/Music) to be moved to an application-specific folder that MediaMonkey for Android has rights to (e.g. /Android/data/com.ventismedia.android.mediamonkey/Music).

2) Read-only access to the secondary storage (for some devices running Android 3.0 - 4.3)

Devices running versions of Android prior to KitKat (4.4) do not have support for Application-specific content, and a small number of them (e.g. Xperia devices running Android 4.x) may not have any workaround to this restriction.  In such cases, MediaMonkey for Android will indicate that MediaMonkey has read-only access to the secondary storage, and the only real solution is to bypass the restriction (see below).

For more information, see:
http://www.androidpolice.com/2014/02/17/external-blues-google-has-brought-big-changes-to-sd-cards-in-kitkat-and-even-samsung-may-be-implementing-them/

Bypassing Write Restrictions to Secondary Storage

The simplest way to bypass these limitations is to update your device to Android 5 or higher, in which case you can bypass these restrictions as described here.

But if you're stuck with Android 3 / 4, and limited access or read-only access restrictions are problematic for you, you can avoid them by using internal memory for your media or via the following options:

1. If you're using an HTC device, configure it to automatically share the SD card via Settings -> Storage -> Primary Storage -> Automatic Switching
2. Use USB Sync instead of Wi-Fi sync (to get content onto the device, though MMA won't be able to edit it)
3. Use alternative firmware such as Cyanogenmod which doesn't have these restrictions
4. Root your device and edit the permissions for writing to external storage.
   1. Edit the permissions as follows:
      o Download and run the following app:
      https://play.google.com/store/apps/details?id=nextapp.sdfix
      
      OR
      
      o Edit the permissions manually
      1. Using a root-enabled file manager, navigate to /system/etc/permissions
      2. Edit platform.xml and find the WRITE_EXTERNAL_STORAGE permission.
      3. Make sure that there are groups in this permission for both:
      
      group gid="sdcard_rw"
      group gid="media_rw"
      
      4. Save the changes to the file.
      5. Restart.
      
      
   2. Reinstall MediaMonkey for Android OR Update to a new version. On running the new version, MediaMonkey for Android will check for new storage locations.

Related articles:

Android 5 / 6 : Can't edit / sync content to the SD Card