Why would you lose anything?

I'm not suggesting removing the old IBM. Just make it the secondary drive. For most intents and purposes it won't make any difference really. The WD drive will fill up first (assuming it's bigger) until the drives have equal space left. Then the software will alternate between drives until they are full.

And then if you wanted to go to 1 drive, assuming the WD has enough space, you can move all the files from the old drive to the new drive while on the empeg. Not super fast, but not super slow either.