Chosen Solution
Hi there, I still have OSX 10.8.5 on my internal drive but mostly boot up from an external drive with Yosemite 10.10.5. The only reason I’ve kept 10.8.5 is because of DVD Studio Pro and Compressor, because I still need to author DVD’s and High Sierra won’t allow me to open those apps, and Sierra on an external drive isn’t recognised as a bootable startup. So, am I able to copy my current10.8.5 to an external drive, then update to Sierra or later, and then have the option of rebooting from 10.8.5 on the external drive when I need to, or will Sierra not recognise it. Many thanks.
I would use Disk Utilities from my internal drive then erase and reformat the external drive. Use the write zeros option and make on pass to map out any bad blocks on it. Format it GUID. Now install your 10.8.5 on it. Later you can reboot from it, use Migration Assistant to move you data onto it. Now you can install Sierra on it.
