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What is the purpose of the hidden files from OS X on my FAT32 external drive (only viewable in Windows)

Sometimes when my wife works on photos in Photoshop on her Mac, what she gives me back are all the photos I took, with their .jpg extensions in place, as well as a set of "ghost" files.

That is, for every photo, there is an evil twin whose file name is the same ... except that it begins with "._". Also this ghost file are sort of "grayed-out" and don't open when double clicked. I suspect they are hidden files, (I have set windows to show all hidden files) but what find of hidden file -- on that, I'm clueless.

I just delete these files and never have a problem with the photo (.jpg) files, but still I'm dying to know; what are they?

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marked as duplicate by Daniel Beck, random Dec 31 '10 at 19:46

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2 Answers 2

Ignacio's answer is partially correct. "._" can store both resource fork information as well as HFS+ metadata, like file type, creator code, and color label information, and possibly extended attribute information, etc.

In the case of Photoshop, it can be set to save a preview of the image that's saved, this is usually in the form of an 'icns' or'PICT' resource in the file's resource fork. (This feature can be turned off in Photoshop's preferences). Additionally, it also saves HFS+ file type and creator code ('8BIM', 'JPEG', etc. file type, though this varies depending on the image type, and a '8BPS' creator code). As far as I know, these cannot be turned off.

They are grayed out because they have their Windows invisible bit set. You should be able to tell from the file size what's contained in them. If the file size is only around 8 KB or so, it's just the HFS+ metadata; if it's larger (64 KB or more), it's a preview of the image. The ._ part is really only useful to Macs, and then only if the corresponding non-._ part of the file is there as well. So if you are deleting the original file from the disk from within Windows, you can delete the ._ version as well.

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The file's resource fork on filesystems that don't support them.

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