[dnanian]

We already do. The "What's going to happen" section will tell you if it's clear the data might not fit.

But, because we don't know what your copy script might or might not copy, nor do we know exactly what's on the drive and what the effect of the copy method might do, it's a crude approximation.

To "really" tell you, we'd have to simulate the entire copy...and that would take nearly as long as a typical copy..

Note, too, that the reason for a failure is specified - it's in the log. (And we give the sizes of the volumes in the pop-ups.)