Can I boot from a USB stick containing MacOS if there is an issue with MacintoshHD? So my mac HD started playing up and then refused to work, would not boot.
Tried recovery mode but it wouldn't allow me to run first aid on the Macintosh HD drive.
Created a MacOS USB boot up stick by formatting a blank USB stick then using the recovery option to install MacOS on it.
Plugged it in, rebooted and held the command (cloverleaf) key.
Mac then displays the drives from which you can boot
Chose the USB stick and not MacintoshHD.
and it is taking forever.
It took me through all the usual options &quot;set up appleID&quot; and so on which I either skipped or chose default.
Then it just went black with the beachball.
Now it's just black with the arrow/pointer.
Doesn't appear to be doing anything..
So is it because MacintoshHD (internal HD) has issues that the USB booter is not able to finish?
EDIT: additions in italics
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.