The entire repository can be moved to a large drive, span across drives (this is harder to maintain, but can be done and is not something I would recommend that a user attempt so as to insure that all data remains centralized and within the same repository location but it is an option), and you could just archive and remove projects that are no longer necessary. You could always start a new project and migrate some of the current data and back up the older unnecessary data as well. You would need to asses and decide what works best for your corporation (also know that the more active projects that aren't being used does slow the system a little as well as it is using resources).
The archival would be started at the main parent folder (it contains the project name, for example ALM_DEMO), and we would just archive this folder and all of its children folder as they are currently positioned to maintain the correct hierarchy of the project. Then we would need to backup/archive that project's schema from the DB, and this would create an entire backup of the project. I would recommend that this be done with all users out of the system, all their entities checked back in if using version control, and the project offline, as this would insure that nothing is happening on the back end by way of clean up jobs to possibly change the data within during the backup/ archival process.
As for the recommended best practice, although HP recommends archiving and backup, there is no recommendation from them regarding the actual best practices and that determination is left up to the end user to determine what is best for their corporation and meets their individual requirements for safety, retrieval, and other requirements that may need to be met such as FCC, FDA, or other government agency systems of check and balances.
Some other valuable ''Eye on Testing'' links that offer different clarifications and helpful tips and options that we like to recommend but these too would be based upon user need and desires are those such as the following:
I hope this answers your questions and provides more insight,