format Formats a disk to accept Windows files ... TIP: You can resize a Partition even if Disk Management fails using DISKPART and FSUTIL command-line tools for Disk Management.

That's the whole process of creating a new partition using DiskPart. You can create multiple parts of the disk by following these steps again. Just ensure that enough unallocated space is available.

The external drive might be in RAW format. Write-protection could be ... A personal suggestion would be to exclude external drives while using the Diskpart utility, but if you have to use it ...