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I have a LaCie external hard drive that I'm using on both Windows 7 and on Mac OS X. I've noticed that when I create files on the drive while it's plugged into my Mac, when I bring it back into Windows those files have some type of hidden files associated with them.

They typically start with a period and an underscore, then have the same exact file name, such as ._filename.ext and are faded out gray so the OS doesn't recognize them.

What are these files for? Are they necessary to keep? Are they taking up extra space on my hard drive? Can I just delete them from the drive while I'm on Windows?

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2 Answers 2

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Files beginning with a ._ represent the resource fork of a Mac OS X file. HFS+ (Mac OS X's file system) allows a file to have different "forks"; FAT32 only supports one fork (or, you might say, doesn't support forks at all) so these resource forks get turned into separate files on FAT32.

The resource fork of a file often just contains metadata, so you can delete them, but doing so could "mess up" some files, so it's best to leave them alone.

For what it's worth, few new Mac OS X applications use resource forks; they were used extensively in the pre-OS X days, but most newer apps have other mechanisms available to them (such as file bundles).

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This isn't quite right -- they're AppleDouble files, and will contain resource forks (of files that have them), type & creator codes (ditto), extended attributes (ditto), and other Mac-specific file properties. Also, while resource forks are becoming less common, extended attributes are becoming more common (e.g. for quarantine data on downloaded files), so ._ files are not going away. –  Gordon Davisson Aug 12 '10 at 20:11

Ignore them or try to find a way to hide the files in Windows (without moving or renaming them). Windows often creates files like these too so it goes both ways.

In Mac OS X you can use "chflags hidden FILENAME" atleast. Don't know how or if you can do something similar on Windows.

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