I can't paste the error because I just broke my loop devices and so can't recreate it any more. But it was recreatable. You get errors because it accesses these files: /lib/modules/2.6.10-gentoo-r7/kernel/fs/squashfs/squashfs.ko /etc/mtab on these lines of the ebuild: modprobe squashfs || ewarn "no squashfs modules - test not completed" umount ${T}/squashfs_dir The second can be fixed easily by passing -n to umount. The first requires you to either skip the test if sandboxing is enabled, or addwrite (don't know how that works though).
I've actually disabled the testing, sine it seemed easier to do then to troubleshoot, and I personally don't have the time to perform a proper fix.