@
elemenoP - following on from
Bobioden's suggestion, this may help you identify what's using space -
http://www.iclarified.com/entry/index.php?enid=14356
@
mbovenka - if the OP put the 'borrowed' books into separate libraries based on loan expiry date, and then removed the libraries when the loan period expired, then the library folders would still be on the disk.
BR