Page 1154 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

to a mean time to failure (MTTF) that is sometimes represented in the
number of times it can be reused or the number of years you can

expect to keep it. For example, some tapes include specifications
saying they can be reused as many as 250 times or last up to 30 years
under ideal conditions. However, many variables affect the lifetime of
media and can reduce these estimates. It’s important to monitor
backups for errors and use them as a guide to gauge the lifetime in
your environment. When a tape begins to generate errors, technicians
should rotate it out of use.

Once backup media has reached its MTTF, it should be destroyed. The
classification of data held on the tape will dictate the method used to
destroy the media. Some organizations degauss highly classified tapes
when they’ve reached the end of their lifetime and then store them
until they can destroy the tapes. Tapes are commonly destroyed in
bulk shredders or incinerators.

Chapter 5 discusses some of the security challenges with solid-state
drives (SSDs). Specifically, degaussing does not remove data from an
SSD, and built-in erase commands often do not sanitize the entire
disk. Instead of attempting to remove data from SSDs, many
organizations destroy them.

MTTF is different from mean time between failures

(MTBF). MTTF is normally calculated for items that will not be
repaired when they fail, such as a tape. In contrast, MTBF refers to
the amount of time expected to elapse between failures of an item

that personnel will repair, such as a computer server.