problem when erasing unused space on flash drive

I deleted files from Flash drive. I want to make them unrecoverable. I tried to wipe free space with Eraser and Ccleaner software. Only data partition was wiped. But both softwares stopped working suddenly. They simply turned off. Eraser sometimes shown error of un-existent device and sometimes stopped with no error.CCleaner shown un-existent device error also. I used 2 passes erasing. The flash surface is Ok. No bad blocks. It was checked with Hard disk sentinel. Recuva siftware normally scans …

ccleaner data device drive error files flash flash drive free privacy problem software space wipe working