Migrating from TS-210 to TS-231P2 with a broken drive

I've got an old TS-210 with one of the two drives having failed. I'm now trying to migrate the data to a new TS-231P2 with new hard drives. How do I best go about this? I read that I should be able to migrate everything simply by plugging the old drives into the new NAS.

My thinking was as follows:

- Install two old drives in new NAS, one of them broken

- Start NAS

- Remove broken drive

- Insert new, larger drive

- Expand existing volume to include new, larger drive

- NAS mirrors data from old to new drive

- Once finished, swap out old (good) drive for 2nd new and larger driver

However, when I do this, the TS-231 does not seem to want to start up - the status/HDD LEDs are green, but I cannot access the web interface. The web interface only works when I either unplug all drives or insert one of the new (blank) drives. Do I just have to wait for a very long time? Thanks!