HathiTrust storage architecture has two synchronized instances of storage with wide geographic separation and an encrypted tape backup with 6 months of previous-version retention.  For more information, see https://www.hathitrust.org/technology.
The University of North Texas maintains redundancy for all the content in the UNT Digital Library system.  For every file there are at least two copies (both preservation files as well as all of the access files that are getting viewed online).