When information is written to a hard disk drive it is stored in a file reminding book's contents. On computers with DOS and Windows the information is stored in a File Allocation Table, or FAT, or in a Master File Table, or MFT. When file is deleted, FAT or MFT tables are updated and tell the computer that there is a free space on the hard disk drive, although in practice data is not deleted until it is not overwritten with new data.
To make sure that you have more or less deleted your data, please use the following:
Use information cleaning tools.
To prevent your personal data from being recovered by third parties, make sure to use special software for that purpose.
Hard disk drive demagnetization:
Demagnetization is a process in which magnetic carriers are erased with opposite magnetic field that encrypts and mixes electronic data and therefore makes it unreadable. Unfortunately one needs to have a special equipment to do so and that is why it is not reasonable for an ordinary user.
Physical destruction of a hard disk drive.
Here it's all pretty clear, just knock yourself out and grind the hard disk drive to the dust and trash it. But in this case you won't be able to sell it anymore.
And finally, I would like to say that if you submitted your computer to the service center and the latter informed you that your hard disk drive is broken and data cannot be recovered, make sure to collect it back and buy a new one.