Where is my free space gone?

Question

I am very new to Ubuntu. I just set up a Hyper V installation on my Windows laptop, and gave it a 30GB hard drive space.

I got a notification that my root drive is almost full. According to the screenshot below I only have 12GB, almost at maximum capacity. I know the Ubunutu installation itself takes up some space, but 18GB is missing. Any ideas?

I've read some similar posts here, for example, somebody had to post the output of df -h --total . So here it is, in case it helps:

Filesystem      Size  Used Avail Use% Mounted on
udev            2.9G     0  2.9G   0% /dev
tmpfs           595M  1.4M  594M   1% /run
/dev/sda1        12G   11G  725M  94% /
tmpfs           3.0G   42M  2.9G   2% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           3.0G     0  3.0G   0% /sys/fs/cgroup
/dev/loop1      3.8M  3.8M     0 100% /snap/gnome-system-monitor/100
/dev/loop4       55M   55M     0 100% /snap/core18/1074
/dev/loop3       43M   43M     0 100% /snap/gtk-common-themes/1313
/dev/loop0      150M  150M     0 100% /snap/gnome-3-28-1804/71
/dev/loop2      1.0M  1.0M     0 100% /snap/gnome-logs/61
/dev/loop5       15M   15M     0 100% /snap/gnome-characters/296
/dev/loop6      4.2M  4.2M     0 100% /snap/gnome-calculator/406
/dev/loop7       89M   89M     0 100% /snap/core/7396
/dev/sda15      105M  3.6M  101M   4% /boot/efi
tmpfs           595M   16K  595M   1% /run/user/121
tmpfs           595M   52K  595M   1% /run/user/1000
/dev/loop8       68M   68M     0 100% /snap/sublime-text/85
/dev/loop9       55M   55M     0 100% /snap/core18/1754
/dev/loop10      97M   97M     0 100% /snap/core/9436
/dev/loop11     1.0M  1.0M     0 100% /snap/gnome-logs/100
/dev/loop12     384K  384K     0 100% /snap/gnome-characters/550
/dev/loop13     2.3M  2.3M     0 100% /snap/gnome-system-monitor/148
/dev/loop14     2.5M  2.5M     0 100% /snap/gnome-calculator/748
/dev/loop15     162M  162M     0 100% /snap/gnome-3-28-1804/128
/dev/loop16      63M   63M     0 100% /snap/gtk-common-themes/1506
/dev/loop17     256M  256M     0 100% /snap/gnome-3-34-1804/36
total            24G   12G   12G  52% -

I am not 100% sure what to do with this, it just confirms my problem. The Avail column says that there are 12GB available somewhere. Is this 12GB locked up in the system somewhere?

edit: Output of lsblk:

NAME    MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
loop0     7:0    0 149.9M  1 loop /snap/gnome-3-28-1804/71
loop1     7:1    0   3.7M  1 loop /snap/gnome-system-monitor/100
loop2     7:2    0  1008K  1 loop /snap/gnome-logs/61
loop3     7:3    0  42.8M  1 loop /snap/gtk-common-themes/1313
loop4     7:4    0  54.4M  1 loop /snap/core18/1074
loop5     7:5    0  14.8M  1 loop /snap/gnome-characters/296
loop6     7:6    0     4M  1 loop /snap/gnome-calculator/406
loop7     7:7    0  88.7M  1 loop /snap/core/7396
loop8     7:8    0  67.6M  1 loop /snap/sublime-text/85
loop9     7:9    0    55M  1 loop /snap/core18/1754
loop10    7:10   0  96.5M  1 loop /snap/core/9436
loop11    7:11   0   956K  1 loop /snap/gnome-logs/100
loop12    7:12   0   276K  1 loop /snap/gnome-characters/550
loop13    7:13   0   2.2M  1 loop /snap/gnome-system-monitor/148
loop14    7:14   0   2.4M  1 loop /snap/gnome-calculator/748
loop15    7:15   0 161.4M  1 loop /snap/gnome-3-28-1804/128
loop16    7:16   0  62.1M  1 loop /snap/gtk-common-themes/1506
loop17    7:17   0 255.6M  1 loop /snap/gnome-3-34-1804/36
sda       8:0    0    30G  0 disk 
├─sda1    8:1    0  11.9G  0 part /
├─sda14   8:14   0     4M  0 part 
└─sda15   8:15   0   106M  0 part /boot/efi

Answer 1

You've got unused disk space. it's like having a 100G disk with a single 10G NTFS partition.

Your whole disk size is 30G. Your main partition is /dev/sda1 where Ubuntu has been installed on which is 12G.

You can create a new partition from your unused disk space and use it as your home to free-up some space. Or use partitioning tools like gparted to resize your root partition size.

sda       8:0    0    30G  0 disk     <---- Your virtual DISK
├─sda1    8:1    0  11.9G  0 part /   <---- Your first partition where ubuntu
├                                           is installed. (12G)
├─sda14   8:14   0     4M  0 part 
└─sda15   8:15   0   106M  0 part /boot/efi