For example, I had a customer that was experiencing systemic tape errors on hundreds of tapes. Specifically, it was related to "edge damage" where the servo track was crimped, resulting in the tape head not tracking properly. The damaged tapes were rendered effectively useless. Although it was initially thought that perhaps it was a manufacturing defect, in fact I eventually discovered that the offending tapes were part of a stack of tapes that fell over.
Issues such as these, in addition to the loss of tapes, wear-and-tear and seek/mount times is why backup to disk, and more specifically backup to deduplication to disk with replication is becoming a more viable alternative to backup to tape.
This was first published in May 2008