Setting up my external 5TB USB3.0 Time Machine disk I decided to try 2 approaches: 1. First I enabled TM on the empty drive that was formatted with HSF+ (non encrypted). I configured TM to encrypt my backup drive on the go. The total size of my TM backup is more than 3TB and this process was taking an eternity. After 2 days the diskutil cs list command was showing that the drive conversion progress was just 20% and TM was still running its first backup work. I decided to cancel that process (was not thinking on waiting a whole week for the conversion to take place. I then took route number 2: 2. I used Disk Utility to erase my TM backup disk and formatted with HSF+ but this time with encryption. I provided the password and after this the drive showed up on Finder. I then enabled TM backups on this drive and this time encryption was selected by default. The whole backup process took this time something like 22 hours. I have also tested with other external disks and this has shown me that it appears to always be much more faster to format a volume with HSF+ encrypted and then put files on it instead of encrypting the disk with all the files already on it. At the beginning I was thinking that full disk encryption would take the same amount of time using erase+format with encryption within Disk Utility and using just right click + Encrypt disk from Finder with files already present on the disk. I had understood that full disk encryption always wrote over the whole disk (files and empty disk space) so it didn't matter if the disk had 10MB or 2TB of data on it, what mattered was the size of the volume, but after these experiments I have found that it is definitely much more faster (almost instantaneous) to just erase and format the disk with encryption from Disk Manager and then copy the files into it. Have you seen this behavior too? You can check the encryption progress by using diskutil cs list on Terminal. Thanks for your feedback.