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Where is data stored when Synaptic scans DVDs?




As I have limited bandwidth, I install from DVDs.
Also I frequently experiment with various options by doing a fresh install to a different partition.
It is annoying to keep re-scanning a set of 13 DVDs.
Where is the information stored?
/etc/apt/sources.list is obvious, but where is the data about contents of each DVD stored?
TIA