Q: Why the 'Create partition' option is unavailable when I right click the unallocated space to create partition?
A: Please check if you already have four primary partitions on your hard drive.
A MBR based hard drive can only have four primary partitions or three primary partitions plus one extended partition (where you can create multiple logical partitions).
If there are four primary partitions on your hard drive and you want to create new partitions, we suggest you to convert a primary partition to logical and then create new partitions.
Note: please backup your data before converting the partition(s).