Mac – Encrypted Time Machine backup is super slow using an encrypted USB volume

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I have an external USB3 disk connected to my MacBook Pro. I first formatted it with GUID/APFS and encryption, and used the "atto disk benchmark" app to measure performance. It showed about 100MB for read/write.

Then I turned to Time Machine, and selected that volume as encrypted backup drive.

But Time Machine writing the backup is super slow. It takes about 100 seconds for 1 megabyte to be written. Thing is running for 20, 30 minutes by now, and I see 17 MB out of 112 GB of progress. It is so slow that Time Machine won't even give me a "time remaining" information.

Is there anything I am missing? Or should I just forget about using such kind of drive for an encrypted backup? (Doing an encrypted backup is a hard requirement.)

Best Answer

Based on the comments by user Tetsujin and this apple community entry the "answers" are:

  • Yes, a full disk encrypted TM backup to a USB3 drive is slow. Overall performance varies greatly, many small files need much more time compared to fewer large files. Overall, my 126 GB backup took about 12 hours.
  • The one thing that you can (and should) take care: format the whole disk as "Mac OS Extended (Journaled, Encrypted)". If you don't select Encrypted, TM will later encrypt the drive on its own, which requires (again!) plenty of time (besides the time it took to just write the backup itself). My initial approach of using APFS: don't do that!

Note: the one other thing I learned: you can easily suspend that first TM backup, simply by clicking the "skip this backup" menu item while it is running.

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