40GB of thumbnails in a 16GB device I just found that my /sdcard/DCIM/.thumbnails folder grew to more than 40GB. Two .thumbdata3 files each large 20GB.

My Galaxy S3 has only 16GB of internal storage. These files prevent me from backing up my sdcard to SFTP.

I think I can safely delete these files, but where do they come from? And how to check if there are any strange ghosts left?
The problem was ultimately fixed by ignoring the whole /sdcard/DCIM/.thumbnails directory in the synchronization.

Deleting the sparse thumbnails was useless as they will grow again but are basically waste. Another interesting point should be telling FolderSync not to sync hidden directories (all caches are hidden) but that will not save directories such as Boldbeast Recorder's .BBRec folder or, in general, any data folder incorrectly marked hidden by the creator app.