My three partitions for my system are created with LVM on a LUKS partition (dm-crypt). These are /home, / and swap. The filesystem is ext4. They are encrypted, because they are on my laptop and I don't want that some laptop thieves get my data. But I often share my laptop with other people so they can access my encrypted partitions. I don't want that these people can recover my cache and all the data I deleted.

So I'm now trying to wipe all my free space on /home to prevent against recovering with tools like photorec. (one overwrite should do, the need of multiple overwriting is just a rumor) But still I haven't found any solution to wipe this free space successfully. I tried dd if=/dev/zero of=/home/fillitup bs=512 count=[count of free sectiors] so my partition was complete full of data. df /dev/mapper/home said 100% is used and there are 0 sectors available. But I could still recover gigs of data with photorec, although I selected to recover just form the free space. photorec displays: /dev/mapper/home - 340 GB / 317 GiB (RO) , but df displays that the size of /home is just 313G, why are there these differences and what did the 340GB means?

It looks like there is a place on my /dev/mapper/home partition, that I can't access to overwrite, but I can access it to recover. I also checked for corrupted sectors, but there aren't any. Maybe this is the space between my existing files?

Did anyone knows why I can't wipe my free space with dd, and how I can find the location of the loads of recoverable files, to securely delete them?