When you do a 'quick format', it "deletes" the files but they remain on disk for future data recovery. If you do a 'slow format', it will delete and erase each sector one by one, then writes '0' on it. This makes it difficult if not impossible for data recovery softwares to extract data from the disk.
If you want an additional layer of security, try using Bleach Bit. It is a a forensic recovery proof data wiping software and it is open source: https://www.bleachbit.org/
There was a famous incident with Bleach Bit where Hillary Clinton's tech guy asked reddit for a secure data wiping recommendation and was recommended Bleach Bit. This post was was discovered and Bleach Bit was discussed in court and eventually the deleted data couldn't be retrieved for investigation, thus stalling the case. Full story: https://www.bleachbit.org/news/bleachbit-stifles-investigation-hillary-clinton
