This is one of those Windows Storage Manager moments where the error message is technically "true" but completely misleading in context.
What Disk Management is really saying is:
"I can't extend this volume with the unallocated space you have, because it's not in a usable position for that operation."
It has almost nothing to do with total free space.
Windows Disk Management can only extend a partition if:
The unallocated space is directly to the right of the target partition.
There are no partitions in between.
So even if you have:
150GB unallocated
300GB free on disk overall
It still fails if the layout is like:
[ C: ] [ Recovery ] [ Unallocated 150GB ] [ D: ]
or:
[ C: ] [ D: ] [ Unallocated ]
In both cases:
C: cannot extend into that space because it's not adjacent.
Why Windows makes this confusing?
Windows reports:
"not enough space available"
when it actually means:
"no contiguous extendable region exists"
It's a UI-level limitation, not a disk capacity issue.
Another possibility
The partition you're trying to extend might already be a dynamic volume. Windows can behave weirdly here, especially if the volume layout is fragmented or if the operation would cross certain boundaries. I've seen Disk Management refuse perfectly valid extensions on dynamic disks while third-party tools handled them fine. Check whether the disk says Basic or Dynamic in Disk Management.
