Dot Underscore ._ files in Dropbox

Recently, all my files stored in Dropbox suddenly acquired a "dot underscore" file in addition to the original file, as shown in the screenshot above:

As a result, the total number of files in dropbox doubled from ~45,000 to ~90,000! To make matters worse, all the "._" files were then immediately synced to my other computers with Dropbox installed.

So why were all these files suddenly created after years of using Dropbox without any issues?

Well "._" files are created to store extended information that would normally be stored as an extended file attributes within modern filesystems such as HFS+ (Apple native), NTFS (windows) or Unix/UFS. In other words, these files are only created on filesystems that do not support extended file attributes - which in most cases means FAT, FAT32 or exFAT.

In my case, I had purchased a Transcend JetDrive 12GB SD card for my Macbook Air to extend the storage capacity to 256GB, and it had arrived pre-formatted with the exFAT filesystem. When I then moved my Dropbox folder from the internal drive (formatted as HFS+) and on to the JetDrive, OS X had created "._" files to store the extended attributes for each file. These were then synced up to Dropbox automatically.

The solution was therefore to...

  1. Move my Dropbox files off the JetDrive
  2. Reformat the Jetdrive as HFS+ ("Mac OS Extended - Journaled" as it is called within OS X).
  3. Move the files back onto the JetDrive
  4. Run the dot_clean command against the JetDrive :

    dot_clean /Volumes/Transcend/Dropbox

The dot_clean command merges all attributes stored within the "._" file back into the master file and then removes the "._" from the filesystem. The fact the file has been removed is noticed by Dropbox, which then means they are removed from all synced copies on other computers (and the web interface).