Ancient technology. Disk packs were used in the
mainframe days of yore when 50 megabytes was considered huge and 500 megabytes was considered colossal.
They were actually a primitive form of removable disk. You essentially had a large cylinder containing ten to twenty hard disk platters, which was inserted into the drive through a door on the top. The drive was about the size of a washing machine, and since it was top loading, they were frequently called washing machines by mainframe operators.
The disks were so massive and spun so fast that they frequently had to be encased in plexiglass so that if the rotation were disrupted, the shattering metal shards wouldn't severealy injure or kill people in the data center.
These machines were also extremely sensitive to dust. Not only was it important to keep the data center as clean and dust-free as possible, but the swapping of disk packs was generally kept to a minimum, as this is when the most dust ended up getting into the system.
The closest analogy to a disk pack nowadays are removeable disk drives such as the Iomega JAZ drive and the Castlewood Systems ORB drive. Both of these use removeable hard disk platters that must be well protected from even the smallest dust particles.