Where every rule comes from

CISA, Data Backup Options

Paul Ruggiero and Matthew A. Heckathorn, US-CERT (now the Cybersecurity and Infrastructure Security Agency), 2012. Its 3-2-1 rule: "3 – Keep 3 copies of any important file: 1 primary and 2 backups. 2 – Keep the files on 2 different media types to protect against different types of hazards. 1 – Store 1 copy offsite (e.g., outside your home or business facility)." cisa.gov PDF

How the site counts: copies = the original plus each backup you list. Kinds of storage are counted across the original and the copies, in five groups: the computer's own drive, removable drive, network / other computer, cloud, disc. "Away from home" = another building; cloud services always count as away; a second drive inside the computer always counts as on the computer. This grouping is the site's reading of "different media types".

CISA, #StopRansomware Guide

"Maintain offline, encrypted backups of critical data" and "Test backup procedures on a regular basis." Used for the "no offline copy" and "no restore test" notes. cisa.gov

Rules of thumb (the site's own)

• Restore test older than 180 days: test again about every six months.
• Work at risk per schedule: every hour = 1 hour, every day = 24 hours, every week = 7 days, every month = about 30 days, "now and then" = no limit you can count on. The newest copy sets the figure.
• A sync folder copies deletions and damaged files too; a second internal drive shares the computer's fate.
• Drive Sizer: room for older versions = files x 1.5 ("some") or x 2 ("long history"); sizes rounded up to common drive sizes. Drive makers count 1 TB = 1,000,000,000,000 bytes; Windows shows binary units, so a 2 TB drive shows as about 1.82 TB.
• Upload Clock: a backup usually gets about 80% of the line's rated speed (you can change it). Typical sustained copy speeds: USB 2.0 drive 35 MB/s, USB 3 hard drive 120 MB/s, USB 3 / USB-C SSD 400 MB/s. Real speeds vary with the drive, the files and the computer.

Official backup and restore help

• Microsoft: Back up and restore with Windows Backup
• Microsoft: Restore files deleted from your OneDrive
• Apple: Back up your Mac with Time Machine
• Apple: Restore items backed up with Time Machine on Mac
• Apple: Restore your Mac from a backup

Sources read on 10 October 2026.