How do I know which macOS my Mac supports?

Check Apple's support page for your Mac model, or use Internet Recovery (Cmd+Option+R) which automatically downloads the latest compatible version.

Can I downgrade macOS?

Yes, but it requires erasing the drive. Create a bootable USB with the desired macOS version and do a clean install, then restore data from backup.

Mac Prohibitory Symbol (Circle with Line) — Boot Volume Not Compatible

Criticalboot

Overview

The Mac prohibitory symbol (circle with a line through it) during boot means the startup disk contains a macOS version that is not compatible with this Mac hardware.

Key Details

The prohibitory symbol replaces the old flashing question mark folder for some boot failures
Indicates the macOS on the startup disk is incompatible with the Mac's hardware
Can appear after macOS upgrade attempts, disk swaps, or migration from a different Mac model
The startup disk is found (unlike question mark folder) but the OS cannot run on this hardware
Common when moving a drive from a newer Mac to an older Mac

Common Causes

macOS version on disk requires newer hardware than available
Disk was migrated from a different Mac model with incompatible OS build
Failed macOS upgrade left an incomplete or incompatible installation
Boot volume file system corruption preventing OS verification
T2 or M-series security chip rejecting the OS installation

Steps

1Boot to Recovery Mode: Cmd+R (Intel) or hold Power button (M-series)
2In Recovery, open Disk Utility and run First Aid on the startup volume
3Reinstall macOS from Recovery: this installs the latest compatible version for your hardware
4If Recovery does not work, use Internet Recovery: Cmd+Option+R (installs latest compatible macOS)
5Create a bootable USB installer with the correct macOS version for your hardware