I am using a 2TB external usb disk as my mac boot disk.
I don't know how that affects the normal upgrade and migration.
However, in general, the thing many people seem to forget is that 10.6 was the last Mac OS version to allow older PowerPC programs (like Office 2004 or older versions of Adobe Creative Suite) to run in emulation mode. Once you get to 10.7, that support goes away and the older programs will not run at all.
If you have "mission-critical" programs that you need for school or creating income, upgrade them before leaving OS10.6.