Can you screenshot your Disk Management MMC? Open it by running "diskmgmt.msc" or typing "Create and format hard disk partitions". Like this:

if you have two drives you cant extend, in disk partition you can nuke that partition, idk if you shrunk a some of your d drive, a screenshot would help out alot more in what you are trying to explain

This is the built in disk management that comes with win 7 pro

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I got the 48GB unallocated partition by chopping a bit off the D: drive, but it won't let me expand the C: partition you can see in the screenshot the option to expand in the context menu is disabled.

I can't just nuke it because the C: partition is where the OS is and I'd rather not go through the hassle of reinstalling windows.

You'll need to use a third-party partition manager to merge that partition with the unallocated space. The Windows built-in tool will only allow extending partitions into free space on the right. I'm not sure if this is because the data is physically separate on the platters and I couldn't find a good explanation.

I've used Aomei Partition Assistant to merge partitions like this before and it was OK. Google will find you plenty of options as well.

Aomei Partition Assistant worked!