All you need to do is open the disk in disk manager, delete all the partitions, and create a new one. Or use diskpart utility and do a clean command on that external drive to wipe all partitions so you can create a new one.
You will more than likely have to use the diskpart utility CLI as @hang-the-9 suggested. In my experience when windows lock partitions on a drive and you need it back you have to use the CLI diskpart tool, windows installer partition select and create GUI tool or a bootable tool for hard drives. Here is a good guide https://www.windowscentral.com/how-clean-and-format-storage-drive-using-diskpart-windows-10 .It didn't work upon running disk management. It still says 30 GB when it's a 1 TB external