How to transfer media (a little more complicated than this...)

I'll try and make this simple.

I have 2 identical projects on 2 seperate drives.

Drive A has 100% of the media. Drive B only has 50% of the media.

For example, in my sequence on Drive A, everything is online, but on Drive B, that same sequence has about 50% of the clips offline.

How can I copy only the media from Drive A that is missing in the sequence in Drive B?

Any ideas?

Geoff