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My Ubuntu server is set up to emulate a TimeCapsule (after a very long weekend following the instructions here, here and here). My macbook pro has been backing up happily to it for a month or so now, and all seems well.

The other day I was tidying up the extraneous files from my music collection on the server, got a bit loose with the find command... and ended up deleting all the .AppleDouble files underneath '/', which included the Time Machine folder.

Now, Time Machine still appears to work fine, it backs up regularly, I can look through all the previous versions of my files, and they seem to restore without trouble. My question is: by deleting the .AppleDouble files, have I actually broken anything? Is the TM data still good, or should I trash it and start fresh (i.e. with a new 'day 0' full backup)?

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up vote 2 down vote accepted

".AppleDouble" files are part of a two-file data format. (They aren't directly related to the Time Machine backup format, they're just data files of yours that are in the backup.) If you delete one of the two files, you've corrupted or lost at least some of the data. Whether or not this is critical depends on the type of data.

See AppleDouble at Wikipedia for more information.

AppleDouble files contain Finder meta-data and the resource fork (if any). The corresponding file in the pair contains the data fork.

In some cases, the data stored in the .AppleDouble file isn't critical. A plain text file has all of its data in the data fork, so throwing away the matching AppleDouble file won't destroy the text data. However, if, for example, the file "owner id" was set so that it would open with a particular editor, that information is now lost, so it will open with the default text editor and the file's icon will be different.

In other cases, all the important data is in the resource fork and the data fork is empty.

You could leave your backup as-is as long as you remember that if you were to restore any AppleDouble-format data in the future, you'd only get half of the "file" and it's possibly corrupt. Keeping the backup around means at least you have everything else available.

Erasing the backup and starting over would guarantee you wouldn't unwittingly restore corrupt files, but then you will no longer have the other contents of the backup.

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Great answer, thanks Chris! – Jon M Sep 4 '11 at 8:37
Chris: how do you tell whether or not the file contains critical data? Is there a way to figure out what's being stored in there? – l8nite Jul 23 '12 at 5:17
l8nite: That's a little involved to answer in comments. I suggest you post that as a question. – Chris Page Jul 24 '12 at 0:00

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