What’s the best way to securely wipe an old internal HDD before selling it?

A SATA-to-USB adapter or external drive dock will work, but make sure it provides separate power—3.5-inch desktop hard drives usually cannot run from USB power alone. If your working PC has an unused SATA data port and power connector, connecting the drive internally will generally be faster and more reliable.

For a traditional spinning HDD, a single full-disk overwrite with zeros is normally sufficient for selling it. In Windows, you can use DiskPart’s `clean all` command after carefully selecting the correct disk. It overwrites every sector, but it can take several hours depending on the drive and connection speed. Double-check the disk number first, because the command is destructive.

If you don’t intend to reuse the drive, physically destroying the platters is an option, but that obviously makes it impossible to sell. Since you want to resell it, connect it properly, erase the entire disk rather than just deleting files or formatting a partition, and verify that no usable partitions or data remain afterward.

A bootable wiping tool such as DBAN can also erase the drive, and an external dock makes the process convenient. Multiple overwrite passes usually aren’t necessary for a modern HDD; one complete pass is generally considered adequate for normal resale. Don’t use this advice for an SSD, since SSDs should be erased with their built-in secure-erase or sanitize function instead.