Turned Off Macbook During El Capitan Installation

You can boot from an external hard drive, or put the MacBook Pro in Target Disk Mode holding the 'T' key at startup.

That makes it an external hard drive to another Mac. In the future always do a complete backup before any install, and preferably have at least two backups.

A backup is not a backup, unless it is an active backup of something else. Once the hard drive has to be erased, the first backup becomes the original, and you hope the second backup is in good condition. For more alternatives see my FAQ*:

http://www.macmaps.com/backup.html

* Links to my pages may give me compensation.

You have two choices:

1. an external hard drive with OS X on it will boot your computer and from there you can start the process of downloading OS X and installing it on the computer’s hard drive. If you go this route make sure the OS X version on the hard drive is at least the same version that originally shipped with your computer. In other words if your computer came with 10.7 and you have a friend with an external drive that has OS X 10.5 on it, that’s not going to work.
2. Buy or borrow a USB keyboard - you can get generic keyboards really cheap. A PC keyboard will work. The Windows key will work as Command and the Alt key will work as Option. Now you can restart your computer in Internet recovery and download/install OS X again.

You can always install the same version of OS X (or newer version of OS X) onto a computer without erasing any files. The OS X installer does not automatically erase the hard drive.

A USB keyboard requires no special driver for basic functionality. Granted, some 3rd party keyboards have drivers that provide added functionality but to simply be able to type etc no driver is required. I’d not have mentioned the possibility otherwise.

When I said using an external drive was an option I didn’t mean that you’d be able to download OS X using your computer - since the keyboard isn’t working you can’t force it to start in recovery mode. But what you can do is find a Mac using friend and borrow their external drive or borrow their computer to create your own boot drive.

IT is simple

1. Reboot computer holding command R to put computer into recovery partition
2. if you are doing this on your computer with a USB keyboard Download OSX and install. That simple
3. if you are making a bootable external choose disk utility and format the drive GUID partition and OSX extended journaled
4. download os x
5. when the download finishes the installer will run. The installer lets you change the target disk.select that option and select the external. Install