macOS 10.15.5 APFS Bug Affects Bootable Backup Drives

SSD hard drive

An APFS bug was found in macOS Catalina 10.15.5, the most recent operating system update for Macs. It affects bootable backups created by software such as Carbon Copy Cloner, the company that discovered the bug.


The bug has no effect on existing backups or the filesystem itself, it remains limited to when a bootable backup is first created.

The chflags() system call can no longer set the SF_FIRMLINK flag on a folder on an APFS volume. Rather than fail with an error code that we would have detected, it fails silently – it exits with a success exit status, but silently fails to set the special flag. That’s a bug in the APFS filesystem implementation of chflags – if a system call doesn’t do what you ask it to do, it’s supposed to return an error code, not success.

SSD hard drive

It’s unknown whether this is a bug or something Apple did intentionally, as Mike Bombich points out in his blog post.

3 thoughts on “macOS 10.15.5 APFS Bug Affects Bootable Backup Drives

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.