Susan Lawlor writes in with a common problem:
I’m trying to securely erase my poor sad iMac before donating or recycling it. It’s my old workhorse—running 10.6.8. Disk Utility’s Erase Security Options is grayed out. I have no OS X disk, and there’s no Recovery HD.
It’s admirable to erase your system before you sell—especially with secure erasure—to avoid leaking personal data to someone who buys it or obtains the disk drive. While the odds are likely very low someone would be able to extract data (or be interested in it), you can try to reduce those odds to what is effectively zero.
Susan has a number of options of how to proceed, but there’s a bit of navigation along the way. She’s running 10.6.8 (Snow Leopard), which didn’t include Recovery HD. Security Options in her version of Disk Utility is grayed, because you can’t erase a disk from its startup volume. (That option is always unavailable for SSDs, but an older iMac won’t have an SSD.)
Read more here:: How to erase a Mac that seems to lack erasure options