MacBook Pro

Encrypted Time Machine Backup – Which is the best method?

This question is related to a previous question that's been locked @

I've decided to keep a time machine backup @ the office since I have a hub on my LG Ultrafine Monitor and will leave a drive connected permanently to it.

When selecting the disk there's a checkbox for encrypting the Time Machine Backup, this option actually encrypts the entire disk, and the way it does this is converting the disk to CoreStorage. This process is independent from the backup itself and will run in the background, encrypting the entire disk.

I've noticed that after the Time Machine finishes the status on the Time Machine Status Bar Icon will show the message "Encrypting" and will show the % of completion. This seems to be taking ages but I'm not sure how much time it will actually take.

I've also noticed that if I format the disk as encrypted before doing the backup it will not convert the disk and the background process will be shown as completed when using "diskutil cs list" command (see previous question that I linked). But also the backup takes really long (it says 21 hours).

Seeing how at the end both approaches seem to take 20+ hours, I'm curious about which method takes less time. My questions and concerns about this are the following:

  • If I use a non-previously-encrypted disk, will it encrypt the entire disk (even the empty blocks as suggested in one of the answers to the linked question)?
  • Would this mean the encryption will take much more time as opposed to using an already encrypted disk?
  • Would both methods take the same time? (which I guess would be the case if only the actual files were being encrypted)

If both methods take the same time, then I think it would be better to use a normal unencrypted disk and let the encryption take place in the background, because:

  • The actual backup will finish faster and then you can eject the disk and finish the encryption later.
  • You can reverse encryption if you wish later, since the disk was forward converted and this allows you to use the "diskutil reverse" command.

So far it seems that when using the already encrypted disk, files are encrypted on the fly when the backup is taking place and that's why it takes more. On the other hand, if the disk is not encrypted, a conversion process will take place but the backup will proceed to write non-encrypted files that will be encrypted later by the conversion process. But if the conversion process is actually encrypting every block of my disk, even empty ones, then it seems to me that it could take MUCH more time!

Any info regarding this will be very welcome, I'm currently doing the previously-encrypted-disk method (already regretting aborting the background encryption and re-formatting the disk as encrypted), so I'll come back after it finish and report the results…

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