This issue duplicates another issue.
Windows: Architect 126.96.36.1996's "Package" Commands fail when Project ist not on C:\
If you do "cd x:\targetdir", also do "x:" to really change to that working path.
Since I could not edit my post, I'd like to add:
I created this Thread since "Build 670 with Cmd 250 is not working on Windows - Not changing working directory" derailed into some Android issue and thinking about wrong backslashes.
Nothing is wrong with the backslashes (Well, actually it is, but Windows 7 is smart enough for this). This really is about Architect NOT changing the drive by issuing "z:".
If I type "z:" and repeat "sencha package build ios.json", everything works.
In DOS, "cd z:\somedir" only switches to that dir in given drive. Your working drive does not change. You have to do that yourself. I guess you are all using a Mac (I exclude Linux because even more is wrong with your build stuff, there).
Good call! I already have a ticket open for this where I'll both window-ify the slashes and add the /d switch