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1. Go to your phone's Settings > Storage

2. Unmount SD Card (your external SD card)

3. Download/install from the app(s) from the Store again

4. Go back to Settings > Storage - then Mount your SD card again (or reboot)

Or you can try this

1)Insert the sd card in you windows pc

2)You will find a folder named android_secure in it you will find the file named smdl2tmp1.asec delete it

(Make sure in pc folder options, the show hidden files are enabled and the hide protected operating system files is unchecked)

Source: XDA