Bug#627488: No problem when using the businesscard.iso
Ran into the same problem with the debian-testing-amd64-netinst.iso.
Specifically, the installer screen goes red when trying to install the
busybox package, the log shows indeed that it cannot find the source
image, while it is still mounted. I should add that I am installing in a
VM from a downloaded .iso file -- tried both a Parallels VM and a
VirtualBox VM, same problem in either one of them.
I then switched to the debian-testing-amd64-businesscard.iso, and now
the installation proceeded flawlessly on both VMs (although the
Parallels Tools don't yet work and produce a black screen and numerous
Xorg error messages -- but that's another story).