You can run a free OS pretty effortless, but when wanting 100% free software, you have to dig deeper and replace the proprietary BIOS firmware.
From Wikipedia, the free encyclopedia
Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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The disks still have proprietary firmware, as do several other components though.
I bet that wireless mouse probably has some code in it.
If you're using an active thunderbolt cable, you wire has proprietary code in it.