Format encrypted disk vs. encrypted disk image

Discussion in 'macOS' started by alkalifly, Jul 31, 2013.

  1. alkalifly macrumors regular


    Apr 13, 2004
    I have an external 1TB USB 3.0 drive. I would like to keep some sensitive files on it, that would like to have encrypted, but I don't necessarily need the whole drive to be encrypted. In fact, I would rather have part of the drive unencrypted so that I can plug the drive into a colleague's Mac to access files we are collaborating on, without allowing access to the encrypted files.

    I know that I can create an encrypted disk image on the drive, and keep all of the sensitive files in said image, while leaving the rest of the drive unencrypted. My question, however, is whether I can expect any performance hit accessing the files off of the encrypted image vs. simply formatting the drive with encryption from the get-go and just having the whole drive be encrypted.

    In other words, will access to files on an encrypted disk image be any slower than access to the same files on a fully encrypted disk?
  2. benwiggy macrumors 68020

    Jun 15, 2012
    It's unlikely that one form of Apple encryption on a small amount of data is going to be slower than another form of Apple encryption on the whole disk.

    I suspect any hit is so negligible as to be unnoticeable.
  3. chabig macrumors 603

    Sep 6, 2002
    Go with the encrypted disk image. That's the most flexible option. A less flexible option is to create two partitions and encrypt one.
  4. alkalifly thread starter macrumors regular


    Apr 13, 2004
    Thanks for the feedback, folks :)

    I actually learned something rather interesting while investigating this further. First, I found a free app from the App Store called Black Magic that measures disk throughput (looks like it's from a company that sells video editing software, so this little utility is supposed to let you know what video formats your drive can handle, but I digress), then I tested both a fully encrypted drive vs. an encrypted disk image on an unencrypted drive.

    I was expecting that there would be little to no difference, but I found a rather surprising difference. (note: the Black Magic doesn't seem to give any kind of cumulative average, it just measures disk read for 5 seconds, then measures disk write for 5 seconds, switching back and forth and just giving you the real-time measurements)

    The fully encrypted disk, via USB 3.0, was giving me around 74 MB/s write speeds and about 99 MB/s read speeds. The encrypted disk image (128-bit sparse bundle, as the fully encrypted disk uses 128-bit AES encryption) was giving me about 98 MB/s write speeds and about 72 MB/s read speeds. I ran this several times to be sure, and the numbers were pretty consistent. So, it seems the sparse bundle, compared to the fully encrypted disk, allows for faster writing but slower reading.

    I can't see why this would make any sense, but those are the results that I got. I would be interested to see if other people experience the same thing testing it themselves. When I measured the unencrypted USB drive on it's own, it was getting about 99 MB/s both writing and reading, so it seems that encrypting an entire drive causes no performance hit for reading but a ~75% performance hit for writing, while making an encrypted sparse bundle causes a ~75% performance hit for reading but no performance hit for writing. :confused:

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