Can a stolen external HD Time Machine drive be accessed on another Mac or PC?

If you encrypt the drive, don't ever forget the password – or you may find yourself locked out of your own backup when you need to use it.

Not if it's encrypted.

VERY important point.

You only enter the encryption password up-front and then normally not again, likely for years (as the Mac remembers it) …

… but you WILL need that password to restore the backup to a new Mac.

The TM’s full disk encryption scheme needs to be setup and operating before the data is written.

It’s not that hard to start over.

Booberoi wrote:

So I have to erase current back up and start again to set up encryption? What about all past backups I may want to use - all deleted when I encrypt?

Very possibly. The sources I'm looking at are a bit confusing.

On the one hand, there are the Sequoia and Sonoma versions of this:

Keep your Time Machine backup disk for Mac secure - Apple Support (PH)

Both say:

"When you switch a Time Capsule or network backup disk from unencrypted to encrypted, your existing backups are erased and new encrypted backup sets are created. When you switch locally connected disks (such as an external disk) to encrypted backups, your data is preserved and encrypted."

But the Sonoma version adds that you must "set up the disk again as an encrypted backup disk." (Wouldn't that involve erasing it?!?)

Then there is Other World Computing – Rocket Yard Blog – How to Encrypt Your Time Machine Backups on a Mac. The screenshot in it is of a Time Machine dialog that includes an [X] Encrypt backups option. (Is this something separate from the Finder's "Encrypt" option?!?). One of the comments:

"You make it sound like you can change this feature on an existing Time Machine Backup, but you can’t. You can only enable this feature the first time you set up a backup drive. And if I’m not mistaken, it will also erase the drive to enable encryption. So it will delete existing backups and files."

Since you are concerned about security, have you enabled Filevault on your Mac? Without Filevault being enabled, anyone with physical access to the Mac may be able to access the data on the internal SSD. Filevault can be enabled/disabled on the boot drive at any time.

Again, if you forget the login/Filevault password you may get locked out although Apple does provide a method to reset the password if you do forget it. However, if for some reason that process to reset the login & Filevault password fails, then you will need to start completely over by resetting the Mac & restoring from a backup.

While a 2018+ Mac with the T2 security chip or M-series CPU always has the data on the internal SSD hardware encrypted, the data can technically still be accessed.

If Filevault is not enabled, then with some Macs someone could boot into Recovery Mode, or from a USB boot drive to be able to access the data stored on the internal SSD. Someone could also put the Mac into Target Disk Mode so that your computer would appear as an external drive on their computer & access the files that way.

Here again is an Apple article regarding Filevault which mentions these things generally in passing:

Volume encryption with FileVault in macOS - Apple Support

Edit: With older Macs it is very easy to access the data on the internal boot drive if Filevault is not enabled. With the later Macs & versions of macOS it becomes a bit more difficult, but not impossible.

Of course, the data is only as secure as the password(s) used for Filevault and the macOS user accounts. An easily guessed password does not provide much security for someone serious about accessing the data or accounts.

Time Machine makes rather extensive use of the features of the file system on the backup drive - whether that file system is HFS+ or APFS. Copying the whole structure in a way that preserves relationships that Tome Machine depends upon is rather more complicated than copying the individual files.

I can’t even back up one of my NON-Tome Machine disks that is formatted using APFS without the backup being MUCH larger than the original. (The original has a number of files that share space on the disk due to APFS “lazy evaluation” of file copy operations.)