2 KiB too large "User Capacity" reported for Toshiba MQ03UBB300

[Pro Backup]

Due to ddrescue: Write error: No space left on device, I spotted a difference in the smartctl reported user capacity, and the actual user capacity.

Reported user capacity: 3000592982016
Actual user capacity: 3000592979968
Difference: 2048 bytes

# smartctl -q noserial -x /dev/sdd
smartctl 6.6 2017-11-05 r4594 [x86_64-linux-4.17.10-1-ARCH] (local build)
Copyright (C) 2002-17, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF INFORMATION SECTION ===
Model Family:     Toshiba 2.5" HDD MQ03UBB...
Device Model:     TOSHIBA MQ03UBB300
Firmware Version: JP050U
User Capacity:    3,000,592,982,016 bytes [3.00 TB]
Sector Sizes:     512 bytes logical, 4096 bytes physical
Rotation Rate:    5400 rpm
Form Factor:      2.5 inches
Device is:        In smartctl database [for details use: -P show]

[Pro Backup]

Source of actual bytes:

# lsblk -I8 -b /dev/sdd
NAME MAJ:MIN RM          SIZE RO TYPE MOUNTPOINT
sdd    8:48   0 3000592979968  0 disk

[Christian Franke]

Is there any special (RAID?) configuration active that uses the last 4 sectors for metadata and therefore hides these from the user?

Smartctl calculates User Capacity based on info from physical disk (ATA IDENTIFY DATA words 100-103). The lsblk command uses info provided by kernel (/sys/devices/.../block/sdd/size).

Please check output of this command:

smartctl --identify /dev/sdd | grep '^ 100'

Expected output:

 100-103  -     .        User addressable sectors for 48-bit commands (QWord)
 100-103  .     0xa3b0:5d50:0001:0000  (5860533168)

[Pro Backup]

As far as I know there is nothing special active. The drive is a 3TB USB 3.0 external 2.5 inch SATA hard drive in a white enclosure and produced by Toshiba. The manufacturer article number is DTP230 and EAN code is 4051528209617. According to the box it was factory formatted with an NTFS file system (for Windows).

After taking the drive out of the packaging it has only be used as a target for ddrescue once.

The command output is as expected:

 100-103  -     .        User addressable sectors for 48-bit commands (QWord)
 100-103  .     0xa3b0:5d50:0001:0000  (5860533168)

[Christian Franke]

The SCSI/ATA translation layer in the USB bridge may hide the last 4 sectors for some reason (bug, storing some data in last sectors, ...).

Please check which User Capacity is reported via USB (SCSI) to the OS:

smartctl -d scsi -i /dev/sdd

[Pro Backup]

USB (SCSI) reported User Capacity is the lower (minus 2048 bytes) capacity:

smartctl 6.6 2017-11-05 r4594 [x86_64-linux-4.17.10-1-ARCH] (local build)
Copyright (C) 2002-17, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF INFORMATION SECTION ===
Vendor:               TOSHIBA
Product:              External USB 3.0
Revision:             5438
Compliance:           SPC-4
User Capacity:        3,000,592,979,968 bytes [3.00 TB]
Logical block size:   512 bytes
Physical block size:  4096 bytes
Serial number:        20170830002272F
Device type:          disk
Local Time is:        Sun Jul 29 14:41:50 2018 CEST
SMART support is:     Unavailable - device lacks SMART capability.

[Christian Franke]

USB bridge reports lower User Capacity than physical SATA drive. Smartctl works as expected.