I have very little experience with linux, so maybe this is a dumb question :)
I run Ubuntu 24.04 on a machine, and I had an old HDD in a usb-case which I mounted using fstab. Worked fine, but I decided it wasn’t appropriate for my purpose and removed it (physically and from fstab).
But it still shows up in the file manager? What am I missing?
Nope, it doesn’t work that way. You have to umount it. You could reboot after removing it from fstab, but that’s a bigger hammer than necessary.