# Cells overwritten as “Null” [closed]

Last week an entire section of a notebook was overwritten(?) such that all cells (headers, text, inputs, and outputs) were merely Null. It was in a section of the notebook that I wasn't using at the time, but fortunately I noticed and was able to recover from a backup file without too much difficulty.

Today, I noticed a different, but entire subsection that exhibited the same behavior (I haven't yet recovered my work on this portion).

The only clue that something strange was happening was that last week MMA informed me that an outside program had altered the contents of the notebook, so it was saving a new copy. This happened several time last week when the issue first presented itself. I haven't seen it since then, however, so this symptom was apparently not present for the current case.

Here's what I'm running: OS X 10.9.2 MMA 9.0.1.0.

Hopefully this isn't a red herring, but the notebook does happen to be in Dropbox, which makes me wonder.

Has anybody seen anything like this? Any ideas?

Below is a screenshot of the original case.

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## closed as off-topic by Kuba, RunnyKine, rasher, Oleksandr R., m_goldbergMay 6 at 11:42

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Take the notebooks out of dropbox. The dropbox software updates files if you or anyone else changes it: so if a collaborator is working on a file at the same time as you, it will not know how to correctly merge the changes. –  bill s May 1 at 21:37
@bills I am considering this. I have no collaborators on this file, but I have observed some strange behavior with Dropbox on rare occasion (nothing like this, though). –  Rico Picone May 1 at 21:42
Mathematica (at least version 8 on Linux) doesn't like it is you mess with files in any way that are open in the front end, even in ways that seem harmless. I've gotten Mathematica crashes for committing open notebook files to svn archives (which I'd expect to leave the disk file alone, except for reading it). The basic rule is: As long as a notebook is open in Mathematica, don't let any other software touch the file. Unlike most other software, Mathematica doesn't close the disk file after reading, only after closing the corresponding notebook. –  celtschk May 2 at 13:36
Thanks for the info, @celtschk! I'm going to have to be more cautious now. I'll probably start using rsync to backup manually to Dropbox in addition to my daily time machine backups (I'm quite obsessive about backups---especially with this file: it's my thesis theory!). –  Rico Picone May 3 at 6:06