Restoring One of Two Volumes

I had a problem with the internal hard disk on my iMac, so, I had to reformat the startup disk. I initiated the restore from Time Machine process from the Recovery HD, but, as I have two volumes backed up to Time Machine -- my startup disk plus an external data disk -- I'm wondering if Time Machine is smart enough to only restore the startup disk volume and not everything including the data from the external drive. I assume Time Machine would let me know there isn't enough space to put it all on my startup disk before starting the restore, but, it hasn't.

Will Time Machine only restore the Mac HD volume or will it attempt to restore all the data from both volumes onto my startup disk? If it will restore all the data, how do I make it only restore the Mac HD volume data? Thanks!