I want to use a backup from an external hard drive I used to be able to do this by restarting holding down the OPTION key.
I am using OS 10.11.4
iPad Air Wi-Fi, OS X Mavericks (10.9.3)
It has to be a bootable backup, not Time Machine or simple drag and drop. How did you make this backup?
Boot Using OPTION key:
1. Restart the computer.
2. Immediately after the chime press and hold down the
"OPTION" key.
3. Release the key when the boot manager appears.
4. Select the desired disk icon from which you want to boot.
5. Click on the arrow button below the icon.
As Limnos states the disk must be bootable meaning it was made from scratch or cloned from another bootable drive. Time Machine backup drives will only boot into a Recovery HD.
I have total backups in a 4 TB WD external HD With 470 GB stored. I also have everything backed up in 1 Tb drive..
I don't use El Capitan but with most systems you can go to system preferences and configure which drive to use as boot drive when starting up.
As I said earlier it is the type of backup that is critical. You are not revealing that to us.
I backup with the time machine into the external hard drive I never had a problem like this with Mavericks.
I have also an old G 5 running OS 10.4 which works with an WD external drive.
I also have a souvenir disk of OS 9
I started with apple on a Mac Plus running OS 7.5 twenty four years ago hooked up to Compuserve.
My Mackintosh hard drive
A new iMac will not startup from or run OS 9 or for that matter, any older OS X version that it originally came with.
see > Use the version of OS X that came with your Mac, or a compatible newer version - Apple Support
At best, you may be able to copy over files like pictures and maybe some documents depending on what program created them.
how do you start with an external hard drive backup
