Thanks Joel for your response.
As there had been 10 passes then all the free space was erased then (it did take a long time). Yes, one pass would have been enough but it was my first time and I wanted to see how long it would take. If one stops after 2, 3 or 4 passes or as desired this will then presumably do the job completely. 35 passes does seem excessive but more than one might seem desirable if erasing sensitive data.