The truth is : Formatting does not erase the data from your HDD completely. Not even low level formatting! So how does one ensure that the data is securely wiped from the data devices like a Hard Drives & DLT / LTO Tapes? The two most secure ways to achieve to ensure data is securely wiped is thru Degaussing and thru Shredding / physically destroying Hard Drives.
For almost all commercial organizations, Data Device Crushing / Shredding is the best option ensuring that your data cannot be accessed by anyone. But the truth of the matter is that even after being shredded, in some case, Data can be recovered from shredded devices. There are very few organizations capable of doing so worldwide but to be honest, it is a possibility that some parts of your data may be recoverable even after crushing. But that needs resources and talents of a very high degree and is normally not available with commercial organizations.
If your data is of a such a level of importance that these high level organizations may be interested in it and you want to be 100% secure, the best option is to Degauss your data device and then get it Shredded / Destroyed.