DDS has a shelf life of 10 years and 2000 "passes." In this sense, a "pass" is any time the head passes over a section of the tape. LTO has a shelf life of 30 years and 1,200,000 passes. Since an LTO tape is a serpentine format, the tape passes back and forth over the head as the tape is read from front to back, back to front. It takes 48 passes to read a single tape. This means that the actual usage is set at 25,000 full reads of the tape. There is one reference on Fujitsu's site (a maker of LTO media) that they've actually tested their media up to 1,200,000 full reads, though. It appears to be a pretty solid media type.
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This was first published in June 2002