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