1) Magnetic tapes are sequential access storage devices that are resistant to environmental conditions and easy to transport and store. They were commonly used to store application data but are now primarily used for backups and archives. 2) Data on 9-track tapes is organized into frames containing 9 bits that correspond to a byte plus a parity bit. Data blocks are separated by interblock gaps. 3) Calculating tape length requirements depends on factors like tape density, speed, blocking factor, and file characteristics. Using a higher blocking factor results in a higher effective recording density and lower space needs.