In your lilo.conf file, add this line in the global section:
In you wanted to be able to boot from sda or sdc after starting to boot from sdb you could also add:
# Chainloading section
label = sda
label = sdc
To boot from the HDD which becomes sdb when Linux has loaded, go into the BIOS setup screens and change the boot device to the HDD which becomes sdb when Linux has loaded.
Repeatedly pressing some key as the computer boots is the usual way to "go into the BIOS setup screens". Exactly which key depends on which BIOS your computer has. Del or Delete are common.