In this paper, an optimal switch-on strategy of dual-stage actuation (DSA) for hard disk drives (HDDs) is proposed. When DSA loop is switched on during the seek settling, the control performance can be compromised by saturation of the secondary-stage actuator. To address this issue, we have established a framework to derive the saturation boundary based on maximum output admissible set theory. In addition, the earliest switch-on of DSA is proposed to lower the overshoot and shorten the settling time. Numerical simulations show that our proposed method successfully lowers the overshoot by 59.4% and reduces the 5%-track settling time by 15.9%.