And while I also emply eSATA externals for backups
I also have an internal drive for my OS backup that I disconnect when not actually making a fresh backup or testing it.
I do my system backups as a whole drive "Clone"
My Data which is NOT on my system drive at all is backed up to
an active internal drive, an inactive internal drive (usually unplugged)
and an external drive.
And data worth saving is worth saving several times.