I did a KDE workstation install. HDA is a 540mb drive set up with / & 80mb swap. HDC is an empty drive (540mb) previously formated with /home & /usr. Wiped drive clean. Just left them formatted. Installer complained about not enough space in/ . Went back & checked & the installer never checked HDC for space & never put any partitions on it. After manually partitioning drive it continued on fine.
This issue is resolved in the Red Hat Linux 6.2 installer which is able to make use of multiple drives.