Twice the native capacity of a media is usually given as the maximum possible amount of data that could be written to a tape. It should be noted that this figure is rarely, if ever, achieved. This is due to the varied types of data that are found in most environments. Each data type has it's own compression ratio and therefore no standard is achieved for the total compression ratio that results. For example; database data, executables and graphics files generally compress the least, and regular text and data files will compress the most.
Clean the device per the manufacturer's recommendation and if required, reboot server and recycle power on the drive to clear any error.
In addition, you can cross-check the compression ratio from Windows native backup utility (ntbackup) with hardware compression enabled (select the same type/amount of data to backup). Should the problem persist with ntbackup, contact respective hardware vendor for further clarification.
For reference:
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Title: How to determine the total bytes written to a piece of media within VERITAS Backup Exec http://seer.support.veritas.com/docs/235307.htm
Title: Backups fail to get double the amount of the native capacity recorded to a media, even when hardware compression is being utilized.
http://seer.support.veritas.com/docs/199542.htm
Hope this helps.
