MacOS – High Sierra disk space issues

You probably tried to manipulate a CoreStorage volume with Disk Utility. Core Storage volumes are built after enabling File Vault 2, creating a Fusion Drive or often after updating to Yosemite.

If you don't have a second internal hard disk attached, disk0 is usually mounted to /. In your case disk1 is mounted to / which is a typical hint for Core Storage Volume Group - in case of an internal Fusion Drive disk2 is mounted to / and disk0 and disk1 usually don't appear entering df -h.

Open Terminal and enter diskutil cs list to get all CoreStorage objects in a tree-like view.

Use this comprehensive list of documented and undocumented commands to manipulate your CoreStorage Volume Group.

It seems like the two most likely candidates are a mounted (shared) disk overwriting a location with large files, hiding them from your view, or large (or many many small) files that are held open by a process, but have already been deleted. If you haven't rebooted the computer yet, that's worth a shot, but also booting into recovery mode and running first aid on the disk might help. lsof | grep deleted might also reveal interesting information, if lsof is available on OSX (I assume it is, but I've never looked)

I've also noticed in my own use that if I use a tool like DiskInventory or Grand Perspective and there are other users on the system, they won't show the size of other users files if I don't have read access to them (reasonably so). If you have other users on your system, they may have large files in their home directory you're not seeing.