After formatting a new external hard drive, follow these steps to set it up as an encrypted backup drive to use with Time Machine.
1. Plug the drive into your computer
2. Click "Don't Use" if prompted to use that drive with Time Machine backups. This will only occur if the drive you plug into the computer has more available free space than the internal hard drive itself.
3. Click the Apple menu at top left of the Finder, click System Preferences, then click Time Machine button
4. Tick the box next to "Show Time Machine in menu bar, then tick the box next to "Back Up Automatically"
5. Click "Select Backup Disk..." button, click the drive from within that list, tick the box next to "Encrypt backups", then click "Use Disk" button
6. When prompted, input a password, a password hint, then click "Encrypt Disk" button. If you forget this password, your only option is to erase the disk completely in order to set it up again for Time Machine or use it for other purposes.
7. The drive encryption process will begin:
8. Then the "next backup" countdown will start for the backup to begin. Observe that the drive now has the word "encrypted" below its name.
9. Status will change to "Preparing backup..."
Conditional for MacBooks: Click the "Options" button and tick the box next to "Back up while on battery power" if you would like the Time Machine backup to run while using the MacBook on battery power.
10. Once preparation is complete, the backup will begin and show amount of data transferred and estimated time remaining
11. Quit System Preferences. If the box next to "Show Time Machine in menu bar" was ticked in step 4, clicking the Time Machine icon in the menu bar will show status of the current backup operation, status of the last completed backup, as well as allow you to force stop or force start a backup.
12. Remember to always eject the backup drive within the Finder whenever you need to unplug it. Each time you plug an encrypted Time Machine drive into the computer, you will be prompted for the encryption password that you set up in step 6 above. If the password is not input, the drive will not be mounted to the desktop in Finder, and consequently, Time Machine backups will not run.
Day-to-Day Use of Time Machine
As a general rule, we recommend that you plug in or leave the Time Machine drive plugged in as often as possible. If too much time lapses between backups, it takes Time Machine longer to prepare and complete the next backup, which also utilizes more of your system's resources in the process.
Too many interruptions to backups while they are in progress could lead to permanent data loss on the drive, which can only be remedied by a complete reformat and re-setup of the drive.