You can move the boot files to the unallocated space if you wish.
By default Windows only allocates 100Mb to the "System Reserved" boot partition it allocates during a normal installation, so 450Mb is plenty.
You will however need to give it a letter (in Disk Management) first, because the next step when you "perform action" is to ask you the drive letter to which you want the boot files copied.
Disk Management flags have the following meanings
"boot" = "this is the system you're running"
"system" = "this is where I found the boot files for the currently running system"
"active" (on the first HDD in the BIOS boot sequence) = "this is where I started the search for the boot files"
"active" (on subsequent HDDs in the BIOS boot sequence) ="this is where I will look if I don't find something in the MBR on the first HDD"
so "change boot drive" is going to set the newly lettered 450Mb "active" instead of C.
However, note the fact mentioned in the previous link. EasyBCD will set the target partition "active" and copy the boot files, but it won't alter or remove the original boot files, so you will continue to boot from them (D will still be flagged "system") until you change your BIOS to boot from the other HDD.
Once you have successfully booted from the copied files (confirm that "system" flag has moved there), then you can manually clean up the boot files from your D drive.
Sorry for the tardy reply. I've been busy knocking down a wall and taking up a floor all this week. Haven't been on the PC much. Better half will confirm that new kitchen trumps PC.