A couple of the photos in my photo collection are corrupt, on both my MBA, Time Machine copies and independent external HDD copy. I would like to run some disk scanning software on the internal SSD, external USB drives that I have (and if possible on the NAS, a AFP mounted embedded Linux box). Any free utilities for this - something like chkdsk that exists since the dawn of time on windows? I've googled without success. I'm happy to work in the terminal, and have MacPorts available. Any suggestions? PS. Please don't mention SMART - this is not the same as a surface scan.