If you connect a hard drive that has been used in another computer before, you might find an undeletable EFI partition on it. In Disk Management this looks as follows:

However, the diskpart command-line utility can be used to delete this partition. By the use of the clean command all partition information will be discarded on the selected disk.

WARNING: this will not only delete the EFI partition but all partitions and everything on the disk.

After that you’ll be able to partition the disk in the Disk Management GUI. Or you can do this with diskpart, too:

See the Technet Documentation for more infomation on diskpart.