I've found this link:
http://www.fixya.com/support/t12303591-processandroid_process_media_stopped
An advice I've found (from a Motorola representative, but should apply to any Android device):
Take out the SDcard.
Go to settings--> apps--> manage apps--> all--> media storage and delete chache and data
Reboot your phone and leave it for 5 minutes. The internal media storage will rebuild (this will not delete any data).
Electro, July 2014