A faster way to accomplish this is to do the following:
This will erase unnecessary data, in my case over 1GB.
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Using a disk space analyzer, I was surprised to see that my Android tablet had 1.03GB of photos. This was odd to me, because I knew there were only a couple of photos on the device.
I pinpointed the problem to thumbnails. These are tiny copies of photos used by some apps to more quickly display pictures. Specifically, the 1GB was being taken up by a thumbnail index file, and I didn't need to lose that 1GB to a function I don't use on the tablet. I erased the file as follows:
1. Open a file manager on Android. I use File Manager from Rhythm Software.
2. Ensure that it can display system or hidden files. This is an option somewhere under Settings. In File Manager, tap Menu > Settings > Show Hidden Files.
3. Navigate to \mnt\sdcard\DCIM\ .thumbnails. By the way, DCIM is the standard name for the folder that holds photographs, and is the standard for pretty much any device, whether smartphone or camera; it is short for "digital camera IMages." Another BTW: when a folder name is prefixed with a period, then it is a hidden folder in Android (such as .thumbnails).
4. Select and erase the file that's about 1GB and contains the word 'thumbdata." The exact file name will vary.
After I did this, image viewing apps like Gallery operated just fine, with no apparent slowdown from the loss of this file.
In this way I freed up 1GB on my Android v4 tablet, 1GB on my Android v4 phone, and 750MB on an older Android v2 phone.
Keeping the 1GB Free
Because .thumbdata is a system file, Android recreates it. Indeed, you may find more than one copy in the .thumbnails folder, if you have reinstalled Android or similarly redid the system in some way.
To keep Android from creating the 1GB file anew, we need to create a dummy file that fools Android. In short, we create a text file with a text editor, and then move it to the thumbnails folder. Here are the steps to doing this:
1. Use File Manager to determine the exact name of the thumbnail index file. On one of my Android devices, the name is .thumbdata3--1967290299. Write it down.
2. Start a text editor or word processor on the Android, and then create a new text file.
3. Use the Save As command to save the file in the DCIM folder. (We move it to the .thumbnails folder in a later step.) Save it with the same name at that index file, such as ".thumbdata3--1967290299". Now, depending on the text editor's capabilities, it might not allow the "." prefix or a blank extension. Thus, you might end up with thumbdata3--1967290299.txt as the file name. We fix this in a later step.
4. Exit the text editor, and then switch to File Manager. Now, it is important you use a file manager like Rhythm Software's File Manager app, because it does what some others cannot: it can (a) rename file extensions and (b) access hidden folders.
5. In File Manager, navigate to the \DCIM\ .thumbnails folder. If the thumbdata3 file is there again, erase it again.
6. Move up a level to the \DCIM folder, and then right-click the thumbdata3--1967290299.txt file name. ("Right click" means hold your finger down on the name until a menu appears.)
7. From the menu, choose Rename, and then rename thumbdata3--1967290299.txt to .thumbdata3--1967290299 -- (a) add the dot (.) to the start of the file name, and (b) erase the ".txt" from the end of the file name.
8. Click OK (or Rename) to finish renaming.
9. Right click the file name, and then choose Move (or Cut).
10. Navigate down to the .thumbnails folder, and then tap Paste.
The dummy file will now prevent Android from creating the huge index file.