How to Encrypt External Hard Drive as HFS+

Once you erase the drive, Disk Utility will provide you with the option of HFS+ encrypted.

There is no way to do it in the GUI. You have to use "diskutil coreStorage convert…" in Terminal

In Terminal, type:

man diskutil

for the syntax.

aaassdsdsadsads wrote:

How do I encrypt an external hdd on macOS high sierra without converting it to APFS without erasing everything.

Control-click the volume, then choose Encrypt <"name">... from the contextual menu.

It may take a very long time (expect several days for traditional rotating media of capacity typical of Time Machine backup drives)... much much longer than it would take for you to recopy everything, hence the reason for my reply: "That's the fastest way to designate a new volume as a Time Machine backup drive and provide protection from unauthorized use..."

You can also obtain an additional external backup drive and designate it for Time Machine. One and only one backup drive does not comprise an effective backup strategy anyway. Encrypt it to begin with, wait for TM to complete a backup, and then erase the other one. TM will back up to both of them, and as many additional drives as you wish.

In an effort to duplicate the problem I selected the smallest capacity external hard disk I had available (250 GB), and chose "Encrypt..."

As expected it is taking a very long time. Give it a few days and I'll let you know, but at this point only morbid curiosity motivates me to determine if a workaround is necessary, or even possible. The right way to encrypt a TM volume is to do what I wrote.

Thanks for your response but I do not want to erase my hard drive. That would take a long time for me to recopy everything.

Otherwise, it will take even longer.

Use Disk Utility's Erase function and choose Mac OS Extended (Journaled, Encrypted).

That's the fastest way to designate a new volume as a Time Machine backup drive and provide protection from unauthorized use. All mine are formatted that way.