Manage Your Media - Take Control of Backing Up Your Mac - page 147

If, when it comes time to erase your drives, you still want to maintain
a copy of the old data, use your backup software to duplicate your
versioned backups and bootable duplicates onto your new (and
presumably larger) disks, effectively keeping a single backup lineage
intact.
Securely Deleting Old Backups
When it's time to replace a hard drive completely, you may consider
giving away or selling your old drive. Before doing so, be sure to
securely erase it so that its new owner cannot use a file recovery
program to retrieve all your data! Merely dragging files to the Trash
and emptying it will not erase the data in such a way that it can't be
recovered. (You can choose Finder > Secure Empty Trash to securely
delete files, but this won't affect any files that you'd previously
deleted insecurely.) Even the default Erase feature in Disk Utility
won't do the trick.
Instead, use a tool that can overwrite the entire disk (including free
space, not just particular files) multiple times with random ones
and zeroes. Clicking the Options button in Disk Utility's Erase pane
provides two ways to zero the data.
Other products that can do this include Trash X ($9.99), ShredIt X
($24.95), and TechTool Pro ($99.99).
Think about Long-Term Archive Storage
Whether you keep a single backup drive in service for many years
or periodically move your data to new drives (see If You Archive Old
Backups… , ahead), you should give some thought to the longevity of
your storage medium. Over a period of years, the data on a hard disk
can degrade even if the drive hasn't been used at all, as the particles on
the platters lose their magnetic charge.
In fact, all digital media degenerates over time—although the physical
process and expected lifespan vary, optical discs (such as CD-ROMs
and DVDs), digital tape, and even flash memory can lose data over
a period of years. Although “archival quality” media exists, with claims