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I have a 1TB external hard drive of data that I have backed up from my main computer. To save space and give me the option of putting more data on this external HD, I'm considering compressing all of the data.

Windows 7 has the option of compressing a drive (right click on the drive -> Properties -> Compress drive to save space). Is it worth doing this type of compression vs. using something like 7zip to compress all of the contents of the disk? I don't plan on using the data any time soon, the drive is used solely as a back up for emergency situations.

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I would definitely use NTFS's built-in compression (which is what you're seeing in Windows 7) over a file-level compressor utility like 7zip. Doing so will be much easier to work with because the compression / decompression occurs seamlessly on read / write, and you don't have to worry about corrupt archives or using third-party utilities to browse the contents.

As far as how NTFS's compression stacks up against other filesystem compression methods I am unsure. If you want to stick with vanilla Windows 7 without installing other filesystems I'd say NTFS is going to be your most painless bet.

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NTFS compression isn't really all that great compared to a dedicated compression tool. First and foremost: NTFS compression is inline, which means it needs to be optimized for speed (CPU usage, latency, low RAM usage) rather than maximum compression ratio. Second is the fact that it's packing data into disk allocation units rather than a variable-sized stream. Third is that tools like 7-Zip can do solid compression to take advantage of cross-file similarity. That's not to say that it doesn't have advantages, but they do force its limitations. –  afrazier Sep 12 '12 at 14:30
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If the files you're backing up are small (<50MB) and numerous, then compacting them into an archive with 'quickest' compression will improve disk speed. I've done this before for both reading and writing large media projects, benchmarking showed a 20-100% speed increase, but only for very light or nil compression.

What I'd do:

  1. Compress the drive unless you need to use it cross platform (don't rely on compatibility or standards for backups ever).
  2. Compress as a solid archive with the 'quickest' option selected (I use winrar, so your options may differ. For a speed improvement you need the lightest compression available)
  3. Put an unarchived version of the archiving software onto the disk. Make sure it's the same version you're using. You'll be happy you did this if you ever need the backup.
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Compression level will depend on the type of file you want to backup but results will always be lower than an external compression utility (from lower to much, much lower).

Make a comparison on a few of your typical files. Right-click on any folder or file and check "Compressed". You can then right-click on a file, go to properties and it will tell you the compression achieved in %. Compare the results with 7zip and make your decision.

If you go the 7zip route, I suggest that you backup individual folders as archive files, and avoid zipping the whole drive in one big file.

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