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I usually use shred to delete complete hard drives.

But I was wondering if and how tools like shred, wipe or dd verify that the expected bits end up on the hard drive.

Does a hard drive always check after a write? Or is there some magic in how the magnetic storage thing works that always detects unwritable sectors or incorrect results on writing?

I also use badblocks -w to wipe disks. Sure, it takes really long but what happens is quite transparent and I tend to trust it more.

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    each of those tools will handle operations differently, but DD will generally break on a bad block unless you are using conv=noerror. keeping in mind that an unwritable sector will remain unwritable, what did you want to have happen in those cases? Personally I use DBAN for wiping disks. follow up with a sledge hammer, and you have a pretty strong solution, suitable for most adversaries.
    – Frank Thomas
    Commented May 19, 2021 at 13:20
  • "Does a hard drive always check after a write?" -- In almost all cases, no, the drive will not perform a verify after a write operation. A sector is typically only deemed "unwritable" if its ID record is corrupted, or previous reads were uncorrectable. When a LBA is remapped is up to the drive's controller.
    – sawdust
    Commented May 19, 2021 at 22:19
  • What does "make a read uncorrectable" mean in simple words? Is that done with some sector-based CRC?
    – Joe
    Commented May 20, 2021 at 4:58
  • See superuser.com/questions/1554322/…
    – sawdust
    Commented May 25, 2021 at 0:39

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