FYI
I had this issue today.
After installing BB1 I installed BB2 without a hitch. I selected to install the ROM update.
After a cold start it would do the first setpatch reset but then nothing, just a black screen. I could boot without startup sequence and get into Workbench via the loadwb command at the shell prompt.
I tried to disable the ROM updates and all that but this things a bastard
So I renamed the Amiga OS ROM Update file in DEVS: and renamed the old one to the correct filename, rebooted, all is now back on
This is the only thread I could find on the internet about this issue, I hope that it helps someone else out too
