Can I copy a bootable El Capitan installer USB stick to a hard drive partition and install from there? I am salvaging a mid 2013 MacBook Air 11" that was discarded with a broken screen backlight due to beverage spill and missing its SSD. (I have a very tiny budget right now so I'm only spending on more expensive items when my testing shows the machine might work well.)

I got the Apple Store to test the hardware then tested the computer myself by taking about two days to install OS X from one USB 2.0 flash drive to another USB 2.0 flash drive using an external display.

It seemed to work quite well other than that it ran at about 1% the speed of a normal Mac for anything that required disk access. This made it hard to test in any depth as problems could easily be due to OS X internal timeouts.

I have now purchased a USB 3.0 external hard drive and knowing how slow USB 2.0 flash drives are to install from and to, I'm hoping to use the fast external hard drive for both, but I'm not sure whether this is possible or how.

The external drive happened to come pre-formatted for Mac with two partitions, which to seems ideal.

Can I just copy the contents of the El Capitan bootable installer USB to one of the partitions? I fear I may need to copy it as a drive/partition rather than as a file or folder, since it must be bootable. Does such a copy require special software?

I need to ask the experts here without being able to experiment since I'm unable to justify the purchase price of a magsafe 2 power supply until I'm confident the machine will work well. I am able to borrow a power supply only occasionally from friends.

I realize doing an Internet OS restore via Wi-Fi is another possibility but the Wi-Fi I have access to is both slow and behind a captive portal. So for this question I'm not pursuing that option thank you.
No, you can't just copy the files from one to another drive and assume that the new drive will install El Capitan. There is a special command to create such an install drive, called createinstallmedia which runs in Terminal


Download the OS X installer from the Mac App Store. Quit the installer if it opens automatically after downloading. The installer will be in your Applications folder.
Mount your USB flash drive or other volume. You could also use a secondary internal partition.
Open the Terminal app, which is in the Utilities folder of your Applications folder.
Use the createinstallmedia command in Terminal to create the bootable installer. 


If you're creating from an existing El Capitan machine, please use the following command. 

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app


I haven't personally tried doing this with an external hard drive instead of a thumb drive. But I don't see any reason for it not to work. 

For more details and how to configure the above command (it depends on the OS you're installing to the USB drive from), please refer to https://support.apple.com/en-gb/HT201372