Tiered storage predates my introduction to PCs, but by the time I earned my enthusiast badge, I was using the technique to increase my system performance. I would actually go as far to say that it was a tiered storage model that put my system at the time into the enthusiast category. So, what is tiered storage you ask?
When I first started using tiered storage, it involved the use of an Adaptec SCSI RAID controller running on a pair of ribbon cables that were as long as my arms. The fastest hard drives at the time were Seagate 15,000 RPM X15 enterprise drives that cost around $1000 each. This was a time before flash as we know it today and thumb drives weren't even available. The Seagate X15 offered 36GB of data storage, but the golden setup for the operating system was three or more in either RAID 0 or RAID 5. The SCSI spec at the time allowed for 15 devices, one being the required SCSI terminator at the end of the cable. That allowed for a high number of hard drives in an era when optical disk drives were a necessity and most motherboards only shipped with two ATA ports that supported just two devices per port, a master and a slave.
With less than 100GB of storage for the operating system and programs on 'high speed' 15,000 RPM drives, other drives were added to the system to actually store data. To increase the overall performance of the system, some users added a small 10K RPM drive to store the Windows swap file - we were concerned with random performance all the way back in 2000! Relatively low-cost 10K RPM drives were plentiful when purchased used from PC recycle centers and worked well in RAID with redundancy for keeping digital pictures (often taken on Sony cameras with a floppy drive for storage) and digital music, the new fad.
Here we are today, nearly 15 years later and facing the same problems as we did back then. The fastest storage is the most expensive and has the least amount of storage capacity. Even though the capacity has increased, so has the data we want to store. The technology has changed considerably as well, but the same issues remain - the more data you keep on your storage device, the slower it performs.