If you see the external hard drive which is not recognized by Windows is full of "unallocated space," you'll want to create a new partition on it. This will allow Windows and other operating systems to recognize and use this HDD.

As we know, different tools like Windows Disk Management tool, diskpart commands, Free Partition Magic like MiniTool Partition Wizard, and others can help to create partition.

I recommend you to see this video to do it with Windows Disk Management Tool