Electronic structures and phonon spectra for metal (Hf, Sc, Zr, Ti) diborides are calculated based on the first principles. As shown, the ‘bulk’ calculations for ‘thick’ whiskers (of 10—20 $\mu$m) can be used to explain the experimental results for nanohardness and elastic modulus obtained by means of the nanoindentation experiments. In particular, a theoretical explanation of the low hardness of scandium diboride (H = 19 GPa) at a relatively high Young’s modulus of elasticity (E = 440 GPa) is obtained. Due to the nature of the electron density distribution, the bonding between the boron atoms and the scandium atom has predominantly ionic character. This enables the runoff of plastic deformation (by means of the accumulation of dislocations) according to the ‘slip’ mechanism between the weakly interacting planes of boron atoms and scandium atoms (which alternate in the crystal). This is the essence of the explanation for the low hardness of ScB$_{2}$. The same slipping will be difficult in other three diborides (of Zr, Ti, Hf) because of the strong covalent interaction between the boron and metal atoms.