Otdfbt

In the last couple of days, Ubuntu 19.04 (Disco Dingo) has been flashing at me saying I've got about 360 MB left and my / folder shows a pie chart with 37.5 GB in the middle of it. It is a 3 year old HP ProBook with an 1 TB disk that came with Ubuntu pre-installed

Surely Ubuntu doesn't suddenly take up nearly a terabyte of space?

Output of df -h -xtmp,devtmp,squashfs:

Filesystem Size Used Avail Use% Mounted on
/dev/sda7 38G 35G 989M 98% /
/dev/sda4 487M 0 487M 0% /media/neil/HP_TOOLS
/dev/sda5 868G 113M 823G 1% /media/neil/HOME
/dev/sda1 200M 30M 170M 15% /media/neil/BOOT

Does this help explain the missing GB?

You have a quite small root partition for system files and applications, most of the disk space is in your home partition for user data.

You will either need to clean up the root partition mainly by uninstalling applications, or also removing log and cache files and such things, or you have to modify your partition layout and shrink your home and grow the system partition. The latter will be a better long-term solution, as you're not going to need less system space in the future, naturally.

To modify partitions, you will have to boot into a live system, as partitions can usually not be resized while they are mounted. To do this, you can boot from the Ubuntu installer USB/DVD and select "Try Ubuntu without installing". This gives you a live desktop where you can use GParted to edit your partitions.

As your home partition is nearly empty currently and it does not look like you'll fill it up soon, I'd be generous and give the root partition around 200GB. Even 100GB should be more than necessary, but you have the space anyway.

Note that any partitioning or file system resize operation always comes with a small risk of data loss or corruption, so making a backup of your data is advisable.

Let's see:

/dev/sda7 38G 35G 989M 98% /
/dev/sda4 487M 0 487M 0% /media/neil/HP_TOOLS
/dev/sda5 868G 113M 823G 1% /media/neil/HOME
/dev/sda1 200M 30M 170M 15% /media/neil/BOOT

All four volumes are on that same 1 terabyte hard drive named /dev/sda. You have a 38G root partition, but your /media/neil/HOME is 868G. That's the bulk of the disk. Someone partitioned things this way, and so that's what it is.

Note that under df, the unsuffixed K, M or G units are 1024-based, whereas the size of your 1TB drive is 1000-based. 907 classic gigabytes (907 x 1024 x 1024 x 1024) works out to 973 GB. There is still a bit missing there toward a terabyte; maybe you have a large swap partition, or perhaps more than one? Note that we are not seeing partitions /dev/sda2, /dev/sda3 and /dev/sda6. From the df output, which shows only mounted filesystems, we have no idea how large these are and what they are used for, if anything.

What's wrong with 'du -sh /*' ? It will show you what root folder takes up the space, then you can go from there.
Though I agree most probably your home folder is on the wrong partition.. you could check with 'echo ~neil'

I think 40gb is enough for root file system. 1tb hard drive was partitioned well enough but not mounted correctly. In current scheme your personal files(aside of system files) was reside on the root partition. /dev/sda5 that mounted on /media/neil/HOME should be mounted on /home. I assume the username you're using was Neil. You should make folder on /media/neil/home/Neil and move all files from your home folder /home/Neil to there. Then you should make changes to /etc/fstab to mount /dev/sda5 at /home.