All good things must come to an end, we’re told, and macOS
10.13 was the very end of the original FileVault, Apple’s file
encryption format introduced in OS X Panther 10.3. With the
original FileVault, a Mac encrypted a user’s home
directory and mounted it as a disk image, more or less. When OS
X Lion 10.7.4 appeared, it offered FileVault 2, full disk encryption (FDE), which
protects your entire drive by using a special startup procedure
at boot time that lets you log in to unlock it. It’s much
better than the original directory method, but it required
faster Macs to work efficiently enough.

But what about people who, like Macworld reader Alex, had
legacy FileVault directories still installed? For many
releases, you could use the Security & Privacy system
preference pane: click on FileVault and click Turn Off Legacy
FileVault. But starting in macOS 10.13 High Sierra, legacy
FileVaults no longer work.

The High Sierra installer shouldn’t have let you upgrade if a
legacy FileVault remained in place, since it would be unusable.
(There was a bug during the beta period that required people
with the Sonos app from macOS in order to bypass an error in
installation that said a legacy FileVault was installed.)

But that happened to Alex. He attempts to log into an account,
and is told, “You are unable to log in to the FileVault user
account ‘name’. Legacy FileVault is not supported on macOS
10.13 and above.”

mac911 filevault can t login

Something was askew with a Macworld reader’s legacy
FileVault account: it shouldn’t even be possible.

However, when faced with the impossible, we can’t deny it, but
try to overcome it. The best course of action is to mount his
current computer using Target Disk Mode on another Mac, and attempt to
mount the sparse disk image format containing his home
directory using his account password. (I can’t test this,
because I don’t have an impossible configuration.)

While that won’t restore the account on that Mac, it will allow
him to extract any files. He could then either restart on that
Mac with another account and create a new account to which he
copies the formerly encrypted files or use Recovery to create a
new account.

If someone in this situation has a Time Machine backup, it may
also be possible to find and retrieve the disk image on the
active High Sierra Mac and decrypt that there. Let us know at if you encounter this or
find other solutions—it seems like it shouldn’t happen at all.

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