Can I re-format OS X drive without removing it from my MacBook Pro?

Solution 1:

You don’t need to remove the HDD and no need for second Mac.

1. Boot the MacBook Pro from the USB stick (that you made with OS X El Capitan 10.11)

2. Using Disk Utilities in the Tools menu, you can reformat the hard drive as part of the installation process.

To boot using the USB, restart your MacBook and press and hold the Option key and Select the USB drive to boot.

Note: you are operating (using OS X) from the USB

Solution 2:

There is no need for the USB stick, you can do a clean install from the recovery partition if it's available. Depending on the origin of the USB stick, it might even be a bad idea to use it at all.

Restart into recovery mode, delete the current installation and reinstall the operating system for the same result: https://support.apple.com/en-us/HT204904

Solution 3:

Can I re-format OS X drive without removing it from my MacBook Pro?

Yes.

You need not remove the hard drive from machine A or connect machine A to machine B to do this.

You simply need a bootable (USB) installer for OS X El Capitan 10.11. Once you have the bootable installer handy, simply leave the hard disk into machine A, plug-in the bootable USB installer, and boot off it.

Once booted, simply start Disk utility, erase the hard drive and perform fresh install of OS X El Capitan 10.11 on it.

You can find the instructions to create a bootable installer for OS X El Capitan 10.11 here:

• How to create a bootable installer for macOS

To boot from the USB, simply restart your MacBook Pro with the installer USB drive plugged-in, and press & hold the Option key. You'll be prompted to select the boot media. Select the USB drive to boot.

For detailed instructions, refer to the Use Startup Manager section in the Apple Support article, How to select a different startup disk.

Solution 4:

The other answers describe the simplest and most straightforward way to do this.

However, for the sake of completeness, consider target disk mode.

Target disk mode allows you to boot your Mac into a state where it presents itself to other Macs as an external HD.

First, connect the two computers together with either USB C, Thunderbolt, or Firewire. Then, on the target computer, restart while holding down the T key.

The target Mac will boot to a screen with a Thunderbolt or Firewire symbol. It should appear as an external HD on the connected Mac.

See this Apple support article for many more details.