A friend of mine has an old iMac (the kind with the half-globe base) that she needs to get rid of. Before I’ll let her do that, I want to clean it up and securely erase the free space. She no longer has the installation media and the system does not have WiFi. Now, I’m not a Mac person (since the Mac II days) and don’t know what software to use for this.
Is there a (preferably free) utility that anyone can recommend?
If you are going to recycle the computer (not give it away or sell it)… .The Disk Utility has a 7pass DOD level reformat option… this will insure that NOTHING could ever be recovered… However bear in mind it can take a day or two to run depending on how large the disk is////
Then do the most destructive level of reformat that DiskUtility offers, there really isn’t any secure way to sanitize just “free space”, meaning you’d have to reformat the drive, and reinstall macOS… check with a local Apple Store, they might do it for you (as they have access to older versions of OSX), since you don’t have a copy
If you just do a “standard” reformat, it doesn’t erase anything, just reconfigs all the file tables, marking areas as rewriteable