The ProBook 645 G1 has a better storage medium than most notebook PCs that have been on the market for more than two years. This laptop has an SSD storage medium, whereas most comparable spec portable PCs that have been available for a similar time 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.
This notebook PC's storage medium (SSD) is in fact the best storage medium across all similar spec notebook PCs that have been on the market for around the same time as the ProBook 645 G1.
The worst storage medium amongst comparable types of notebook PC is HDD.