the 3-2-1 rule
As coined by Peter Krogh here
There are two kinds of people in the world – those who have had a hard drive failure, and those who will. Make sure this common occurrence does not lead to the loss of your image collection.
- He recommends keeping 3 copies of any important file (a primary and two backups)
- He also recommends having the files on 2 different media types (such as hard drive and optical media), to protect against different types of hazards.*
- 1 copy should be stored offsite (or at least offline).
*While 3-2-1 storage is the ideal arrangement, it’s not always possible. A second media type, for instance, is impractical for many people in the ingestion or working file stage. In these cases, many people make do with hard-drive-only copies of their data. Best practices, however, still require 3 copies and some physical separation between the copies.