Don't think SOlaris supports booting from an external hard disk.
You can put it in an internal disk.
To boot it make sure its boot loader isn't installed in the MBR. If Solaris is installed in 3rd partition of 1st disk it will be known by Grub as (hd0,2) and can be fired up by commands
title Solaris in 3rd partition of 1st disk or (hd0,2)
The above is using a Linux's Grub to boot up Solaris' boot loader which is also Grub. The former is version 0.97 while the latter is 0.95. The v0.95 inside Solaris is a special version.
All Unix-like systems like Solaris and BSD can be booted exactly the same way as a MS Windows.