You don't need to know many guitar scales ... then strum all six strings. Just as with the C chord, Em is almost identical to a scale, this time the E minor pentatonic scale.

Harmonic minors use the relative major key signature and raise the seventh note of the scale. For example, in A harmonic minor there are no sharps or flats other than the raised seventh of G#.

Listen to this audio extract to hear C Major modulating to A Minor. You can follow ... The difference is that all modes use the same tonal scale but start and end on different notes.