TBB doesn't cache anything on the hard drive. All browsing is cached in RAM for that session and discarded on shutdown. That is by design to protect your anonymity. It would be retarded if TBB cached stuff to disk. However, if you save bookmarks, those will be stored on disk. Also, if you install add-ons, or you configure add-ons in unique ways that you make you fingerprintable, for example if you whitelist a specific combination of sites in NoScript, that will be stored on disk.