Error unmounting /dev/sda: target is busy

 My ssd had wouldn't unmount for unknown reasons, since everything that was using it was closed I was very surprised and annoyed. The error it would give me when I tried to unmount it in console was:

umount /run/media/user/ssd
Error unmounting block device 8:0: GDBus.Error:org.freedesktop.UDisks2.Error.DeviceBusy: Error unmounting /dev/sda: target is busy

So I had to do (following this):

 sudo lsof /dev/sda

That gave me the process that was "using" the hard. It was xdg-docum for a movie that was not playing or anything but apparently, the hard drive thought it was. So after I killed the process I could normally unmount it from Dolphin.

How to find lost hard disk space

 1. Instal ncdu

sudo pacman -S ncdu

2. Clear caches:

yay -Sc

pip cache purge
 

3. Install Filelight

It shows very nicely where is your lost disk space. Mine is obviously in The Witcher 3 and Skyrim. Oh well.

How to create liveusb from terminal

1.  Download the iso (https://linuxmint.com/download.php)

2. Optional, check the sha256 sums. 

3. Plug in your usb. In my case, I had to delete the partitions, but I didn't create a new partition. Confirm the path to your usb with $df  - mine was at /dev/sda

4.  sudo dd bs=4M if=/home/user/linuxmint-21.1-cinnamon-64bit.iso of=/dev/sda status=progress oflag=sync

5. Voila