Here's what you can do to re-create this.
1) build a 3TB volume.
2) snapshot this volume.
3) attempt to have scalr mount this snapshot in the farm role editor.
Since the UI still prompts for a vol. size even though the snapshot is chosen, my gut instinct was to set the size to the size of the snapshot (3TB). This is when scalr threw the error on attempt to save farm.
However, it looks like the error that i've screenshotted here is only a *warning*, as i can still save this farm role, refresh the page, and see my settings appear to stick.
-K