You are right. When I updated my Mk III with v.30 the first time, I used the disc that I burned using the fastest speed, and it didn't work.Next time you need an upgrade, try burning it at the slowest speed. If you have another burner try that also. Not all burners work as well. A bit of trial and error is needed.
My nav just went black screen on me in the middle of installation.
Luckily, I burned the second disc using the slowest possible speed, pop it in the Nav, did the upgrade and it works well eversince.