I wanted to emerge mozilla and it take a lot disk space. I had little free space on my HD, so I plugged another HD and made a symlink from /var/tmp/portage to a directory on it. But when executed "emerge net-www/mozilla" it raised errors like: tar: mozilla/build/package/rpm/SOURCES/mozilla-ns-upgrade.desktop: Cannot open: Permission denied tar: mozilla/build/package/rpm/SOURCES/mozilla-nspr-packages.patch: Cannot open: Permission denied But when removed the symlink and I recreated the /var/tmp/portage directory I received no more of that error, of course the emerge would not complete because of lack of disk space.
edit make.conf to change where PORTAGE_TMPDIR is