I have a new iMac running El Capitan. Bought a SanDisk flash drive for backup. I created an encrypted disk image on the flash drive.

Here's the problem: copying 10GB to the unencrypted part of the flash drive takes about 5-6 minutes. Copying the same 10GB to the encrypted disk image is estimated to take 2 hours.

Why the huge difference in copy times? Is there any way to speed up file copying to an encrypted disk image?


Finder needs to encrypt each file before copying it to the disk.

No, there is not a way to speed it up without removing drive encryption.

  • Are you sure? Is this a change in the OS? For example, copying to an external hard drive USB 2.0 takes the same time whether to an encrypted disk image or to an unencrypted portion of the disk. But the copy speed to a flash drive is very different depending on whether it's encrypted or not. – UncaMikey Jul 6 '16 at 14:01
  • @UncaMikey is the flash drive actually USB 3? In my experience, external HDs have more stable write speeds, and flash drives tend to be much slower. If you're using a sparsebundle disk image, remember that it had to write each band separately, then write the file to it. If you're using a normal disk image, it has to rewrite parts of the disk image. – At0mic Jul 6 '16 at 15:13
  • Yes, the flash drive is a brand new SanDisk USB 3.0 256 GB. Thanks for the info -- I've long used external HDs and small flash drives < 4 GB, this is my first experience w/ large capacity flash drive. Perhaps the problem were my own expectations. – UncaMikey Jul 6 '16 at 15:28

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