unable to merge primary partition with free space

Solution

In order to extend a volume through the built-in disk management utility, there are some requirements. Here's what the documentation says:

For logical drives, boot, or system volumes, you can extend the volume only into contiguous space and only if the disk can be upgraded to a dynamic disk.

To extend a basic volume, it must either raw (not formatted with a file system), or it must be formatted with the NTFS file system.

_{Source: Extend a Basic Volume}

In this case, what you need to do is to right-click the free space volume, and choose Delete Volume from the context menu. The space will become unallocated, and you should be able to extend the volume.

Further reading

• Extend a Basic Volume