to commonly asked questions.

Leave drives connected during an OS install?

Quick question - when installing El Capitan should I keep my external hard drives attached or unattached to my Mac?  Thanks.  I was leaning to unattached.  

I really like this question. It doesn’t really matter at all, but to avoid any confusion, I would suggest unmounting those external drives. It will speed things up a small bit and will prevent you from accidentally installing 10.11 on the wrong drive. The same can be said for when a person uses disk utility to reformat a drive. I like to unmount all other drives while working on one just to avoid any accidental catastrophe.

In the end, it really doessn’t matter however as long as you pay attention to which drive you’re installing the OS onto. You’d have to go a little out of your way to mess things up.
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