I think I found it. gcc-3.4 has only sid. There is no sarge, no testing and no unstable, only sid.
D-I uses sarge/testing to check the mirror and unstable later to install, iirc.
D-i had not asked to select between Release, Testing or Unstable when the "Bad archive error" came up.
What would be your suggestion to install from the gcc-3.4 archive?
Regards
Harri
