secure erase free space on SSHD
I have a 500Gb Toshiba SSHD installed inside my MacBook as a replacement startup drive. Using Disk Utility I have tried to erase/overwrite the free space on this HD. The "Erase Free Space" button is grayed out and hovering gives the following message: "Erasing free space not available on this type of drive".
Is there anything I can safely do to erase free space? Since I have deleted a large number of files from the HD to free up space for future work, I am just trying to be security conscious.
An aside: I have wondered if with a SSHD there is less or even no need to erase free space as there might be no residue in the memory locations to permit recovery like there is with a conventional HD, but this is speculation on my part.
Thanks, John
MacBook (13-inch Mid 2010), OS X Mountain Lion (10.8.5)