Hence, you need to create one or multiple partitions on a physical hard drive/SSD, for example, C drive, D drive, etc. Without partition, unallocated space cannot be used to store files or install system. What does unallocated space mean? When you install Windows, the installer will prompt to partition hard drive and format partition first. Does that mean the unallocated space cannot be merged to C drive?" However, when I right-clicked C drive, the option "Extend Volume" is inactive. In Disk Management, I shrunk D drive and got 100GB unallocated space which is between the Recovery and D drive. C drive is running out of space, so I want to move some free space from D to C. "Can someone here tell me how to allocate unallocated space to c drive in windows 10? The internal hard drive is 5GB and it is partitioned into 4 partitions, they are EFI system partition, C drive, Recovery and D drive in sequence. What is unallocated space and how to make use of it? Bonus tip: How to recover data from unallocated disk space?.Solution #2: Merge unallocated space to an existing partition.Solution #1: Create a new partition on unallocated space.What is unallocated space and how to make use of it?.