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Q: Host protected area (HPA), hdparm, and 4 KB sectors

I have a couple of questions concerning the host protected area (HPA)
on SATA disks.  All the following is no problem with my Linux system,
I just want to understand things:

1. My two Seagate Barracuda 2 TB disks were not bought as part of a
   completely installed computer but separately as single parts.
   I.e. there should be no recovery or installation software or
   similar in the HPA.  However, both drives have a HPA of about 1 MB.
   What could be the reason for Seagate to do that?

      # hdparm -N /dev/sda

       max sectors   = 3907027055/3907029168, HPA is enabled

2. The man page of hdparm warns about changing the HPA using hdparm -N,
   because data loss is extremely likely.  Is that only because of the
   possibility of using the command incorrectly or are there other
   risks, too? Otherwise, I'd like to disable the HPA and see what's
   in that ~1 MB space.

3. Does the kernel or hdparm have a problem with HPA and disks that
   use 4 KB sectors?  I have an external 4 TB USB hard disk that is
   shown like this:

      # fdisk -l /dev/sdc

      Disk /dev/sdc: 3.7 TiB, 4000787030016 bytes, 976754646 sectors
      Units: sectors of 1 * 4096 = 4096 bytes
      Sector size (logical/physical): 4096 bytes / 4096 bytes
      I/O size (minimum/optimal): 4096 bytes / 4096 bytes
      Disklabel type: dos
      Disk identifier: 0xb6217b1c

      Device     Boot Start       End   Sectors  Size Id Type
      /dev/sdc1         256 976754645 976754390  3.7T 8e Linux LVM
      # hdparm -I /dev/sdc | sed /^Cap/q


      ATA device, with non-removable media
              Model Number:       ST4000DM000-1F2168
              Serial Number:      S301JQ3M
              Firmware Revision:  CC54
              Transport:          Serial, SATA 1.0a, SATA II Extensions, SATA Rev 2.5, SATA Rev 2.6, SATA Rev 3.0
              Used: unknown (minor revision code 0x001f)
              Supported: 9 8 7 6 5
              Likely used: 9
              Logical         max     current
              cylinders       16383   16383
              heads           16      16
              sectors/track   63      63
              CHS current addressable sectors:   16514064
              LBA    user addressable sectors:  268435455
              LBA48  user addressable sectors: 7814037168
              Logical  Sector size:                   512 bytes
              Physical Sector size:                  4096 bytes
              Logical Sector-0 offset:                  0 bytes
              device size with M = 1024*1024:     3815447 MBytes
              device size with M = 1000*1000:     4000787 MBytes (4000 GB)
              cache/buffer size  = unknown
              Form Factor: 3.5 inch
              Nominal Media Rotation Rate: 5900

   Interestingly, fdisk shows 976754646 logical sectors of size 4096
   bytes while hdparm reports 7814037168 sectors of 512 bytes.

   But when showing the HPA of that disk I get

      # hdparm -N /dev/sdc

       max sectors   = 7814037168/7801454256(18446744072933654192?), HPA setting seems invalid

   The native max address is 12582912 sectors (exactly 6 GiB) smaller
   than the end of the accessible disk, i.e beginning of HPA.
   Nevertheless, I can dd up to the end of the disk without any

   So what is wrong with this disk?