The ProBook 645 G1 has an SSD storage medium which is better than most portable PCs of this spec and age. Most portable PCs of a similar spec and age to it have an HDD storage medium. 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 actually the best storage medium amongst all portable PCs of a similar age and type to the ProBook 645 G1.
Amongst similar types of portable PC the worst storage medium is HDD.