Just a general question: Why does 7-Zip not support standard PKZIP-Spanned/Splitted files?
Like in Spec Chapter 8.0 Splitting and Spanning ZIP files:
TotalCommander opens that files successfully - 7Zip not?
Edit: Winzip fails too. http://dotnetzip.codeplex.com handles this files successfully
ZIP code is already pretty complicated.
With Splitting and Spanning support the complexity will increase and it will be difficult to support such code.
Did you take a look at the dotnetzip source?