Quoting Craig Sanders (cas@taz.net.au):
The debian installer (and presumably ubuntu and others) let you switch to another console tty with Alt-F2, Alt-F3 etc to get a root shell. You can manually create the partitions you want, then switch back to tty1 to install on the partitions you just created.
IIRC, on debian tty1 is the installer menu, tty2 & tty3 are for shells, and tty4 is a log tail of info and error messages etc printed by the installer.
And very handy all the other virtual consoles are, too. (1994 thanks you for that tip, Craig. ;-> ) Still, I continue to prefer to use a best-of-breed live-CD disk with a very recent kernel (maximal hardware support) and highly reliable and diverse command-line tools for utility purposes such as partitioning and initial mkfs -- a superior environment for that purpose, IMO, than distro installers, even ones I like, like Debian's, are ever likely to furnish. I therefore also recommend that approach to others.