rsync: writefd_unbuffered failed to write 4 bytes

I love rsync. Rsync is the type of command that makes you heart a Unix/Linux-based system. It helps you back-up/sync files; it's easy to use; it's powerful, but not overbearing. It's like chess or caps: a minute to learn; a lifetime to master.

At any rate, I was getting the error in the title of the post. Turns out it wasn't rsync, but that I had filled up my partition - accidentally. Long story short, I created a folder intending to mount is as my external harddrive, but forgot to do the mounting. So, if you encounter this error, it might be because the device is full.