Dmitrijs Ledkovs, one of the current Ubiquity maintainers, responded to this brainstorm idea as part of the "most popular ideas" review:
Indeed, having less hard-drives attached during installations (ideally only 1) makes the installation process easier and more robust. On the other hand, we still want to continue supporting multiple target devices. During 12.10, improvements were made in detecting external USB drives in the installer. Unfortunately, that also reviled that some USB sticks & drives mis-report themselves as other connection types (e.g. SCSI). This makes Solution #2/#3 very hard. We no longer have migration assistant in the installer, thus there is no benefit in having unnecessary devices plugged in.
I like "Solution #1: Display awarning message". It can be shown in a non-intrusive way, as needed
(when extra drives are plugged in), at the prepare step or maybe at some other screen.