Why don't we just buy bigger hard disks?You may ask why, in the light of the cheap prices of hard disks in hardware stores and electronics markets with an enormous amount of storage space, we don't just buy more.
The explanation starts with the fact that the hard disks we have in use on our servers (SCSI/SCA or SAS) play in a different league (also regarding the price) than the unreliable disks sold for home use (IDE/ ATA or SATA). The latter ones are not manufactured for permanent use and cannot be replace in a running system.
You would be probably very unhappy if we had to shut down our system several times a year for hours, only to replace a crashed hard drive.
Further more one may remark that is misleading to consider nowadays cost for hard disk space. The hard disks we have in use were purchased some years ago and will hopefully continue to work properly for some years to come. Every upgrade or replacement involves a lot of work and probably extended downtime.
Last but not least there is a daily backup procedure into the data processing center. We pay for that service according to the amount of data they store for us. On the other hand, a higher volume of data also means that every backup and migration takes more time to be completed, The backup process should not take longer than overnight. Downtimes due to migrations also affect your work time.