Ok so it wasn't seeing any of my already-attached USB disks. I grabbed another, attached it, and this one it sees. But, even after I've made a TM backup onto it, the Migraiton is failing to acknowledge it has a backup on it 
Ok so it wasn't seeing any of my already-attached USB disks. I grabbed another, attached it, and this one it sees. But, even after I've made a TM backup onto it, the Migraiton is failing to acknowledge it has a backup on it