Fixed Block Architecture
Fixed Block Architecture (FBA) is a disk layout in which each addressable record (block) on disk is of the same size. The term fell out of use, since nearly all modern disk drives use this principle, termed logical block addressing and usually having a constant addressable block size of 512 bytes.
Count Key Data
IBM mainframe systems' disks, called DASD, had traditionally been addressable in a form MBBCCHHR (extent-Bin-Cylinder-Head-Record, which was capable of storing records of varied size. Besides direct addressing, programmers were able to search by key, using the underlying count-key-data (CKD) structure.
Hard Disk Drives
- IBM System/360 Operating System: System Control Blocks 
- IBM System 360 Model 20 Functional Characteristics
- IBM System/360 model 44 Functional Characteristics