Possible reasons that lead to "Extend Volume greyed out" issue on dynamic disks:
You are extending a partition that does not have RAW or NTFS file system.
You are extending a logical partition but there is no free space on the neighboring right side.
You are extending a primary partition but there is no unallocated space on the neighboring right side.
Possible reasons that lead to "Extend Volume greyed out" issue on basic disks: However, sometimes you can't extend boot partition Windows 10/8/7 with Disk Management because Extend Volume is greyed out. You just open Disk Management Utility-> right-click the boot partition -> select Extend Volume -> go through the Extend Volume Wizard, then all will be finished in a minute. Windows built-in Disk Management is a good choice for you to extend boot partition because it is free and reliable to use. This post mainly talks about why Disk Management can't extend boot partition and how to extend boot partition in Windows 7/8/10 flexibly. However, in many cases, you will find that you cannot use Disk Management utility to do this work, because the option Extend Volume is either missing or greyed out due to some reasons. For example, there is a need to extend system partition size when encountering low disk space warning in Windows 7/8/10. As time goes on, the disk layout may seem unreasonable, and resizing partition is necessary.