Wiping data from an old Mac
QHow can I wipe all personal files from my old Mac prior to its disposal?
AFirst de-authorise all apps, including Adobe CS/CC products, and iTunes accounts, including opening iTunes and all its content on that Mac. Then make at least two archival copies of anything from that Mac which may be of importance, so that you can retrieve it in the future.
Once that’s done, sign out of iCloud on that Mac, and allow that to remove iCloud data from the Mac; that data will still be accessible to other Macs using that iCloud account. Finally sign out of your account in Preferences of the Messages app.
To erase that Mac’s storage, when running Mavericks or later, your Mac must be connected to the internet. Start it up in remote Recovery mode, with ç, å, and R keys held down. Select Disk Utility, then select your startup drive, and click on the Erase tab. Erase free space too, selecting a level between the two extremes of fastest and most secure.
Erase your storage and set it in Mac OS Extended (Journaled) format, then quit Disk Utility. If your Mac is being sold or passed on to another user, select Reinstall OS X. When that process reaches the step of inviting you to select your country and region, press ç+Q and shut your Mac down. When any new owner starts it up, it should present them with a Welcome screen just like a new Mac, and all your old data will be gone forever. Additional details are at bit.ly/electriclight_old_mac.