SyncTime itself does not use encryption, but you can encrypt the destination volume and protect it with a password. When connecting a password-protected volume, you’ll be asked by macOS to enter the password; you can also store the password in the keychain so that you don’t have to enter it every time when connecting it to your Mac.

Warning: formatting your destination volume will erase all files on it.

1. Open Disk Utility (in /Applications/Utilities).
2. In the main menu, select View > Show All Devices.
3. In the sidebar, select the root of the destination device. (The root usually contains one or more items named like “Container disk1”.)
4. In the main menu, select Edit > Erase…
5. In the sheet that appears, enter a name for the new volume and select an encrypted volume format, like APFS (Encrypted).
6. In the nested sheet that appears, enter the password you want to use to encrypt the volume, then confirm it in the field below.
7. Click Choose to close the password sheet.
8. Click Erase.
9. Wait until the operation completes, then you can quit Disk Utility.