I’m trying to backup /home/user to my backup drive using MC(Midnight Commander) and shortly after it starts I keep getting the Message I’m out of space on / .
Now I have / and /home on separate partitions. I recovered some space by remove stuff using Pamac but what is causing the lack of space in the first place? I even rebooted.
Note I recently reinstalled Manjaro if that helps.
I am guessing that your backup drive wasn’t actually mounted, and therefore, that instead of writing to the backup drive, you were in fact filling up the directory on the root filesystem that is meant to serve as the drive’s mountpoint. 
Sideways related, you might find the following very well written tutorials by an experienced and helpful user (who shall not be named) of some value… 
Honestly theres more than a few places to look if you havent been actively managing your space …
paccache -rvk2
paccache -ruvk0
sudo pacman -Scc
…etc…
You have a relatively mature account ( even existed on the old forums - hello ) … so I assume you have some basic knowledge of how data accumulation and packages and filesystems and things work, yes?
I did in fact mount my backup drive before trying to backup to it.
I’ll go and try those. But yes I do have some knowledge but don’t know what you are referring to as basic.
Ah, but did you mount it in the right place? 
Yes? Of course. MC starts copying then ask me if I want to overwrite or skip then short after I get the out of space message.
Now it seems I have something wrong and no matter what I remove I’m still showing 0 bytes on / . I even rebooted after remover a number a packages on cleared /tmp. Don’t know what I could have done to cause this.
There is no way that copying to an external volume would increase the amount of used storage on your root filesystem. Copying the files involves reading them into the buffers in RAM and then flushing those buffers to the target volume.
Therefore, logic dictates that the target volume isn’t mounted and that you are, in fact, copying all those files to a directory on the root filesystem. 
Yes I’m aware of that. But but I was watching a few movies stored on the portable drive before I tried backing up my files. So yes it was already mounted.
Let me ask you this again… Was it mounted in the right place?
What do you mean the right place? It is mounted on / as it should be. Where else could it have been mounted at?
How can you mount your backup volume on / and still have a working system? 
Sorry it is /portable not / . I’m tired as I haven’t gone to bed yet.
Please paste the output of… 
mount | grep -i portable ; echo ; df -h 2>/dev/null
Nevermind I figured it out. The portable is mount at /run/media/user/portable
with files sent to /run/media/user/portable/user. Of course isn’t my real user name…
I don’t know why I put in /portable in the first place. I backup many before.
In other words, you were copying files to /portable and not to /run/media/whatever?
Yep sure was. Don’t why I did this in the first place. Stayed up past my bedtime.
See? The problem was in the biological unit between the keyboard and the chair all along, just as I suspected.
Aren’t most computer problems caused by that?
Well… statistically… yes.