Why can't I extend this partition?

My system hard drive on my Windows computer is partitioned into 3 primary partitions, and 200+GB additional free space. The partitions are contiguous:

   C:   20GB
   D:   25GB
   E:  208GB
free:  212GB

I'd like to expand the E: partition, but in the Windows Disk Management GUI, the Extend Volume context-menu option is grayed out and unselectable:

Why won't Windows let me expand this partition?

Copied from Facepunch:

Delete that logical partition containing the free space at the end then you'll be able to extend it.

Had the same problem trying to repartition a Vista machine to make have a big enough single partition for Windows 7. Was unable to get Vista to do it, even after deleting the other patition on the drive. Ultimately solved the problem using gparted, as described here.

Try unmounting it first. RClick, choose 'Change Drive Letter and Path', then remove the existing drive letter. Then try to extend.