Can I boot from a USB stick containing MacOS if there is an issue with MacintoshHD?

Solution 1:

Yes. Mac hardware is designed to boot from an external OS whether it’s an external drive, cheap commodity USB Flash thumb drive, network boot, internet recovery or direct attached disks. The startup manager governs which item is selected and you can connect the OS after you power on the Mac.

It even will search for a bootable drive if the last one stored in Non Volatile Ram (NVRAM) isn’t detected.

• https://support.apple.com/en-us/HT201372
• https://support.apple.com/en-us/HT201255 (Intel only startup keys)

In your specific case, you would need to look at the startup logs for the OS to determine the issue or bring another OS to bear to rule out the first OS or drive you’re using. On slow USB media it can take dozens of minutes to boot, but on fast external media, it should be minutes tops.

Also, a failing drive can bring down the Mac and prevent booting from a known good OS so your option might be to remove the drive entirely (if feasible) to test that or have it repaired.