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I have a couple of old Time Machine backups (for computer A and B) that are stored unencrypted on an unencrypted drive. I would like to encrypt them for archival purposes but they need to remain working with TM (e.g. browse using the TM interface, restorable to a new computer using TM and so on).

I followed the instructions at https://www.howtogeek.com/305540/how-to-encrypt-your-macs-time-machine-backup/ but since A and B are long gone I used computer C. That didn't work out very well, at the final step, instead of starting to encrypt I got this message:

screen shot

How can I encrypt an old TM backup when I no longer have access to the machine that was the source for that backup?

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Warning: I haven't tested this procedure, and there's at least one report of this converting the volume to APFS, which might cause trouble. So I can't really recommend this, I'm just posting it for the record.

An encrypted Time Machine backup is just a regular Time Machine backup stored on an encrypted volume (or an encrypted disk image, if it's on a network server -- not relevant here). Therefore, you can convert an unencrypted TM backup to encrypted just by encrypting the volume it's stored on. For some reason, this option isn't available in Disk Utility, but you can do it in the Finder by right-clicking (or Control-clicking) on the volume, and choosing 'Encrypt "volume name"'. You'll need to enter an encryption password (be sure not to lose this!), and then leave the disk attached for possibly quite a while as it converts the entire thing to encrypted format.

Note that @Allan's comments about TM not really being suitable for archiving still apply, whether the backup is encrypted or not.

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  • I don't have that option in Finder. Are you sure? See imgur.com/a/vDN1Au9
    – d-b
    Aug 13 '20 at 21:29
  • This doesn't really answer my questions. As I understand the linked article you can encrypt /Volumes/2TB/Backups.backupdb/A (or something like that).
    – d-b
    Aug 13 '20 at 21:31
  • @d-b Weird, under both Mojave and Catalina it won't let me convert a TM disk to encrypted. I bet it has something to do with it being bootable (TM disks include a Recovery system, in case you need to do a ground-up restore), but I'm not clear on the details. If you have an earlier version of macOS available (probably Sierra or earlier), that should be able to do it. I'll experiment a bit more and edit if I find another solution. Aug 14 '20 at 3:27
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A couple things...

  • You can’t retroactively encrypt a Time Machine backup especially without the source available
  • Time Machine is not designed to be an archival system

Time Machine is a back up tool meaning it examines a filesystem on a a computer and makes copies of files based on certain criteria. In this case, it copies over only files that have changed since the last back up. Additionally, it will back up system configuration files specific to your system. Without the source, Time Machine simply can’t function

As for archiving, you save data, not computer settings. The plist for your program (say Microsoft Word, for example) is useless in the future. Even the software that gets backed up will be OS and system specific. There are apps that worked fine just one or two versions of macOS ago, but now with Catalina, they don’t work. Without the correct system (hardware and and OS) these settings can’t do you any good.

Going forward, what you can do to archive things on an encrypted volume is manually traverse the folders on the Time Machine backup and copy them. The folder structure is the same as what’s on your drive (i.e. /Users/user/Documents), the only thing is that you’ll have multiple copies for each date Time Machine ran. It will be time consuming, but it will achieve what you’re looking to do.

Going even further Forward, it’s worth thinking about how you Structure your files for backup and archival now. This will make it easier to store your files for archival purposes without having to negotiate the complexities of Time Machine.

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    You can’t retroactively encrypt a Time Machine backup especially without the source available - according to the linked article that is exactly what you can.
    – d-b
    Aug 6 '20 at 19:07
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    Time Machine is not designed to be an archival system - what does this even mean? I want to be able to restore A or B to another machine in the future. That is exactly what Time Machine supports. I just restorted a Macbook Air to a new Macbook Pro. Flawless.
    – d-b
    Aug 6 '20 at 19:09
  • And when you read the steps...they are creating a new Time Machine backup on top of the old one - they remove the disk, then add it back. I also state - without the source available since you need the source to actually do this process.
    – Allan
    Aug 6 '20 at 19:13
  • Time Machine is a disaster recovery product. You backup your computer, but you archive data. What time machine backs up is not suitable for archiving anything because you have will have multiple copies of the same document - for every change you made. You asked about archiving and you just described backup/restore They are totally different.
    – Allan
    Aug 6 '20 at 19:17
  • Read the article You can retroactively encrypt your existing Time Machine backup, which allows you to keep your old backups.
    – d-b
    Aug 13 '20 at 21:12
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Do you really need to encrypt the backups themselves?

The simplest solution to protecting them would just be to store them on an encrypted hard disk. If you have a spare external drive, just format it as an encrypted disk using Disk Utility, or turn on FileVault for that volume (via Preferences->Security&Privacy). My understanding is that both of those methods are, for all of your intents and purposes, identical (the only meaningful difference affecting encrypted boot volumes). Then copy your TM backups on to it the encrypted volume. From then on, you can mount the encrypted volume - entering your encryption password, and then use the disk as normal until you're done, and then unmount it.

I guess if your TM backups are on a disk of their own, you could just turn FileVault on for that disk - even simpler/quicker.

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