HP BB118BV - StorageWorks Data Protector Express Package User's Manual & Technical Reference page 146

before overwriting any media. However, to ensure successful data recovery with incremental jobs,
follow these guidelines:
Have at least as many incremental media as there are days between full or differential backup jobs. For
example, if you run full backup jobs every five days, have at least four incremental media; if you run full
backup jobs every seven days, have at least six incremental media.
Never recycle incremental media between differential or full backup jobs. If you run more than one
incremental job in a row, be certain to not recycle any of the media used during this string of incremental
jobs.
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Backup Jobs