Reboot loop with Android (fixed on a Galaxy S4)

[Downunder35m]

I had it now twice on my S4 and once on my Nexus - you do something or a game crashes and after that the damn thing is stuck in a reboot loop.
Usually you will see the logo and the start of the boot, but then it gets dark and the fun starts over and over again.
So far I followed the advice given in several Android forums, which meant booting into recovery mode and to perform a full wipe incl. factory reset.
Although this procedure resulted in a working device without a loop I often missed to create a current backup of my data - after all you do that if you plan on something or the last is a long time ago....
But the last crash with reboot loop made me mad as I just installed a bunch of programs that also downloaded a ton of additional data, even with the last backup several hours of installing would have been wasted, not to mention all the things that had changed since the last backup, like updates.

I stumbled upon a thread where a user suggested just to remove the SD card to get the phone working quickly, especially if the crash happened far away from your backup or PC.
With nothing to loose I removed the battery and SD, battery back in and started the phone.
To my surprise it started with no problems al all, only everything that was related to the SD was missing.
Trying with the SD again resulted in the same loop as before, so I checked the SD with the Windows tools but got an all clear.
Copied everything from the SD to a folder on my PC and did a format with SD formater from SDCARD.ORG.
All files back onto the SD and just to make sure everything is working fine I used the to create a backup.
To my surprise the program failed several times at different points with an error stating the file can not be found.
A check with the Windows Explorer showed the SD is inaccessible.
Replugged the card reader and all fine again.
At this stage I started to suspected a faulty SD card, which was one of the main answers in the Android forums as well, suggesting to replace it...
A test on the PC (after another quick format) showed no problems at all, copying and deleting files, playing videos of the SD all fine.
But once I copied the previously secured files on the SD and inserted it into the phone it got stuck on the boot.
However, when mounting the SD after the phone was started the SD was accepted and the phone worked.
A quick check of the menu showed that one program was grayed out, stating it is not installed.
Another search of possible files related to that program only found.
The folder .android_secure contained a file for the affected program with the ending .asec - I could not get them to show up with any root explorers I tried on the phone, so I used the PC instead.
Deleted the file and installed the program again, after that all worked as before the reboot loops.

The asec files are encrypted files mostly for paid apps and they are checked at the boot, if one (or more) is corrupt you will get a boot loop.
This loop, causes the restart before the reading and writing on the SD is finnished, resulting in a corrupt SD.
I tried to reboot the phone after mounting the SD (without the corrupt file deleted) and got the loop again.

Checklist and procedure in short (if you are stuck in a reboot loop):
Remove the battery when the phone shuts itself off and remove the SD.
Start the phone again without the SD - if it works you might continue, if you still get stuck you might need a factory reset.
Use a card reader to copy all files and folders to a secure location on your HDD.
Do a quick format with the tool from SDCARD.org and copy everything back on the SD.
Start the phone without the SD and once done insert the SD to get it mounted.
Check the menu for any missing program, app or widget.
If indeed a paid program is grayed out, unmount the SD and on the PC delete the corresponding ASEC file from the SD.
After re-installing the faulty app your phone should be back to normal with no data loss.

Every time your device performs and unexpected reboot with a working app there is a chance the SD will get corrupted.
If this is caused, for example by an MP3 or music file the was currently playing you should notice the problem alread when you try to copy the data off the SD the first time as the corrupt file will refuse all copy attempts.
Please make sure that after every stuck start or boot loop you use the backup files after a quick format of the SD, if in doubt you can also format the SD directly from the Android storage manager.
This procedure will not work for all problems but it might help you to avoid data loss or even to replace a SD, that not really is faulty but get's corupted by a faulty file during the start of the phone/tablet.

[Downunder35m]

Just got confirmation form a user in another forum that my way saved his butt.
Copied all data from the SD to HDD, reformatted the SD and copied everything back ecxept the ASEC file related to the program he used at the time of crash.
No wipe, no data loss.
However, I was told that my fix is unrelated to the reboot and shut down issue caused by OS problems as reported with the 4.3 and 4.4 updates.
On the hand I think no matter the reason for the crash, if there are corrupt files after that, and if these files are used during the boot they still might be the cause for being stuck in a loop instead of admiring one random reboot.