no - external drive wouldn't
you should add to the disc set - not just replace it (unless it's faulty)
my system has a small fast HDD for the O/s - and then a couple of big drives for storage
drive1
c: holds my o/s and core software
it's 60% free
drive two - two partitions
d: is my data drive (easy to backup)
e: games and progs that are on trial
drive 3
f: storage
Z: - paging file
I then have some nas space for storage and backup
http://clonezilla.org/
will transfer your existing Os to the new drive