OS X El Capitan - Running Mavericks off of usb drive So I plan to erase my MacBook Pro completely. After I erase its drive from an El Capitan recovery drive that I have, I want to boot into Internet recovery mode, plug in another flashdrive, install Mavericks on it, and then use the recovery drive for El Capitan to install El Capitan onto my MacBooks internal SSD.

Basically, I want to have El Capitan on my internal SSD and Mavericks on my external USB drive. Is this possible at all, to have an older version of OS X installed on an external device?
If you have the Mavericks install app:


Copy the Install OS X Mavericks.app file to your desktop
Insert the flash drive you want the installation files to be copied to
Download this script, unzip it, then move the osx_10.9_bootable.sh file to your Desktop.
Open Terminal app, then type cd ~/Desktop/ and hit enter
Then type sh osx_10.9_bootable.sh and follow instructions then wait


If this was successful, restart your Mac and hold down the Option key then select the newly created installation drive to install Mavericks onto your external HDD.



(in this case, 0553225513 is my external hdd, which will be your flash drive)

After the installation you can reinstall OS X El Capitan on your Macbook.