Are you looking for an automatic (and ongoing) solution? If not then why not just copy the complete set onto the 2nd drive, then delete the incomplete set. After you're synched up you can copy new photos as you get them.
You might be able to use Windows briefcase (seach windows help for info about it) if you want something that's somewhat automatic.
I'm assuming the 2 drives are on the same computer. If not then networking the computers together would probably provide more options. Not that I actually know any of them...
The simple copying or the briefcase should work on networked computers too.
You might take a look at this freeware program.
It compares contents of a folder on one drive to another drive and automatically copies any changed or added files to the other drive. I use it to backup items stored on my local drive to a network drive. Really easy to set up and sounds like it will do what you need.
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