Does the IBM z60m laptop support booting from the external USB hard drive???
If yes then install Linux and the bootloader to this external hard drive and set vthe BIOS to boot first to the external hard drive.
If not then you can still install linux to the external hard drive and install the bootloader to this drive also, but do not set the BIOS to boot from the external hard drive first. Instead follow the instructions found here starting from "You may choose to let another boot loader handle the install";
Note since this is an external USB drive the partition will be something like /dev/sdX not /dev/hdX.
In either case attach the external USB hard drive to the laptop before powering on.