Started with 50 MBps, went down to 20 MBps shortly after and is declining slowly since. Running for 7+ hours.
HDD is 5 years old, rare use but very well kept.

Edit: external 1.5 TB HDD connected over USB 3. Overwriting with zeroes while formatting using gnome-disks.

Update:

Stopped gnome-disks ~78% and continued writing zeros using dd for the remaining sectors.
command used: #sudo dd if=/dev/zero of=/dev/sda1 bs=1M seek=1001250 status=progress (don’t copy without understanding), used seek here to skip already zeroed sectors.
write speed went up from ~14 MB/s to ~100 MB/s.

slow speed could be caused by multiple passes of overwrites by gnome-disks (not sure if it does that), or by “initializing the filesystem at the same time as zeroing” as mentioned by @ares35.

gradual speed decrement was observed in both methods, as mentioned by @Synthead.

Thanks to everyone for being so helpful.

  • Eager Eagle
    link
    91 year ago

    It depends, it might be doing a single pass, but doing several is quite common.

    • Max-P
      link
      fedilink
      31 year ago

      Probably selected secure erase or something which would write a bunch of random and then zero it out.

      Super common in the enterprise world, probably overkill for OP.