Like some others here, my Galaxy S3 battery was draining overnight from 93% to 6%, whilst doing "nothing" !! The culprit in Better Battery Stats was either Media, or mediascanner.

After research here and xda, I suspected the SD Card, and mp3 files/folders. What I did (and I did not root my phone to do this at all :-

Removed all mp3 from internal storage, and remove SD card, reboot phone - battery drain now "normal" overnight c.2.5% per hour. So, it's either the SD Card, the folder structure, or the mp3 files themselves causing it.

Re-Format SD Card in the phone, not on Computer. Put back in phone, hard re-set phone and start from scratch.

On Computer :-

1. Remove all "&" and "_" from mp3 filenames
2. Use MP3Tag to remove all "&" and "_" from mp3 tags
3. Organise into Folders like Artist/Album only, NOT multiple deep nested folder structure like Genre/Artist/Year/Album etc. Ensure no "&" and "_" in the folder names for Artist & Album.
4. Transfer all these folders onto the external SD Card directly under Media, making no more sub folders as explained in 3.

On Phone :-

1. Check SD Card for folder structure, and that no "wildcards" have appeared in the transfer process (unlikely).
2. Review each file that it does bot have a broken symbol next to it, or it may be corrupted.

Re-boot phone, and let the media scanner do it's work for the first time. It should be fairly quick, if the SD Card is not corrupt, the folder structure is minimal, and the folders/files/tags contain no more wildcards.

Disable DRM-Protected Content Storage & Downloads (both Media Related) in your application Manager|All tab. First Clear Cache, then Clear Data, then uncheck Notifications, then Force Stop, then Disable. don't try & clear any other of the Media related data, or you will lose other stuff.

Battery life should be dramatically improved !!

I have gone further and disabled a whole pile of other bloatware that I could without rooting, disabled Motion, also I stopped all the Location Services, and Automatic syncing; I do that manually when I need to.

I have now been getting between 2.9 and 3.7 days on Battery with moderate usage, for 2 weeks or so now, and no sign of Media or mediascanner in my my Battery Stats. Average battery drain overnight with no activity apart from Alarm Wakelocks is between 0.7 to 0.9% per hour.

Hope this helps someone.