Is it possible to boot an IBM T42 laptop from the usb port (using a usb hard drive)?

Answer 1

Actually, it would be under the CMOS settings. You should be able to enter CMOS settings when you first turn your computer on (it will say something like '' or 'Press Del For Setup' Look for something like that).
Once into the CMOS Settings, you may have to have an Administrator or Supervisor password.
Now, look for Boot Options (or sometimes it's simply 'Boot' or 'Boot Configuration'). Go in there and see what you are able to set your boot devices as.
Also, see if your machine allows you to boot from USB or enable USB legacy support. Both should be enabled.
Hope this helps!

Answer 2

go into the bios and see if it has an option under boot order for external drive