The ProBook 655 G1 has an SSD storage medium which is better than the HDD storage medium found on most portable PCs that have been available for more than two years. Solid State Drives (SSD) store data in semiconductors on a circuit board rather than using magnetic or optical media. Solid State Drives have no moving parts which makes them very fast and reliable.
This notebook PC's storage medium (SSD) is actually the best storage medium amongst all laptops of a comparable type and age to the ProBook 655 G1.
An HDD storage medium is the worst storage medium across similar types of notebook PC.