to list your disks, use:
/dev/sda is probably your first physical disk
/dev/sdb is probably your second disk
look at the output of df -Th to see what Linux mounted for you
the different partitions of your main disk will be named
/dev/sda1 /dev/sda2 etc ...
just be sure what you're doing, you don't want to end up deleting the boot partition, or other nasty things