The HP ProBook 645 G1 has an SSD storage medium which is better than the HDD storage medium found on most similar spec laptops that are a comparable age to it. 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.
An SSD storage medium is in fact the best storage medium across all laptops of a comparable age and type to the ProBook 645 G1.
Across similar types of portable PC the worst storage medium is HDD.