Question: Can't turn on encryption on existing Time Machine backup
When I try to turn on encryption on an existing unencrypted Time Machine Backup — following Apple's instructions — I get the following error:
There was an error preparing the Time Machine backup disk “Time Machine”.
POSIX reports: The operation couldn’t be completed. Invalid argument.
- MacBook Pro (13-inch, mid 2012), 16 GB memory.
- macOS 10.13.1
- MacBook Pro storage is an SSD formated as: APFS (Encrypted).
- backup is over USB 3.0 to an external 2.5" HDD.
- external HDD backup drive is formated as: Mac OS Extended (Journaled).
Any suggestion what I could try, in order to turn my working, existing and unencrypted Time Machine backup into an encrypted Time Machine backup?
MacBook Pro, macOS High Sierra (10.13.1)