How secure is a full factory reset?

[felix1]

Hello everybody,
I got a new (more capacity) My Cloud device and now I have a 4TB Version left over.
I plan to sell it but I wonder how secured the ‘Full Factory Reset’ is working. Does it run over the whole device and set everything to zero or what is it doing? regarding to the time it needs (other discussions in this forum talk about nearly 3 days) this might be the case but I could not find any information on this.
Do you have a clue?
Best

(sorry if I got the wring category… I was totally lost…)

[cat0w]

This is from the 1st generation My Cloud. Click on, tap or activate image to enlarge it.

[Bennor]

WD Support is somewhat vague on what exactly is done when one performs a Full Factory Restore.

Western Digital Support | Western Digital

Critical:

• Performing a “Full Restore” on a My Cloud is data destructive and cannot be undone
• Then entire hard drive is securely erased and formatted
• Data Recovery specialist and software CANNOT be used to recover content

Digging through some of the firmware coding (/var/www/rest-api/api/System/src/System/Configuration/Controller/FactoryRestore.php) it mentions “zero-fill” data and shares.

[felix1]

thanks for your answer :-). This helps a lot

[sktn77a]

Writing zeros doesn’t stop the FBI from recovering data from hard drives so I’m sure any decent hacker can do the same, if so motivated.