Best practice to put HDD into laptop before running DBAN?

I finally gathered enough info and files to put ye olde flakey HDD out of its misery. Burned Darik’s Boot and Nuke (DBAN) onto a newly bought 32GB USB stick (as recommended), figured out the booting-from-stick, and DBAN is merrily destructing away at 94MB/s. For 3 passes overwriting a 640GB HDD, this should take 3x1.9 hours = 5.7 hours. Unfortunately, the speed has steadily dropped from 95 to 83MB/s over 50 minutes, and DBAN predicts the time remaining as 9.5 hours (10 hours total).

The only thing I'd do differently next time is to put the flakey HDD into the laptop instead of writing to it over an eSATA cable connecting to an external drive housing. The HDD is supposed to support a SATA speed of 3Gb/s=375MB/s (4x faster than the 94MB/s above). Of course, putting the HDD into the laptop only generates 4x speed if the laptop internally connects to the HDD in a way that supports the full SATA speed. I've been searching Toshiba Satellite A660 PSAW3C-047017, but the machine is probably too old. My geek-fu is not good enough to determine this detail.

Is it best practice to put the HDD to destroy into the laptop before running DBAN?

As a related but tangential question, why does the speed slow down steadily? DBAN's estimated time remaining has steadily increased from 9.5h to 9:51 even though the elapsed time has increased from 27 to 51 minutes.

For reference, here is the smartmontools report showing reallocated sectors, which was the impetus for erasing and recycling the drive.

smartctl 6.4 2015-06-04 r4109 [i686-pc-cygwin-win7(64)-sp1] (cygwin-6.4-1)
Copyright (C) 2002-15, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF INFORMATION SECTION ===
Device Model:     TOSHIBA MK6476GSXN
Serial Number:    Y1DQC0GOT
LU WWN Device Id: 5 000039 3a3f854e2
Firmware Version: GB001M
User Capacity:    640,135,028,736 bytes [640 GB]
Sector Size:      512 bytes logical/physical
Rotation Rate:    5400 rpm
Form Factor:      2.5 inches
Device is:        Not in smartctl database [for details use: -P showall]
ATA Version is:   ATA8-ACS (minor revision not indicated)
SATA Version is:  SATA 2.6, 3.0 Gb/s (current: 3.0 Gb/s)
Local Time is:    Sun Jan 17 20:17:48 2021 EST
SMART support is: Available - device has SMART capability.
SMART support is: Enabled

=== START OF READ SMART DATA SECTION ===
SMART overall-health self-assessment test result: PASSED

General SMART Values:
Offline data collection status:  (0x00) Offline data collection activity
                    was never started.
                    Auto Offline Data Collection: Disabled.
Self-test execution status:      (   0) The previous self-test routine completed
                    without error or no self-test has ever
                    been run.
Total time to complete Offline
data collection:        (  120) seconds.
Offline data collection
capabilities:            (0x5b) SMART execute Offline immediate.
                    Auto Offline data collection on/off support.
                    Suspend Offline collection upon new
                    command.
                    Offline surface scan supported.
                    Self-test supported.
                    No Conveyance Self-test supported.
                    Selective Self-test supported.
SMART capabilities:            (0x0003) Saves SMART data before entering
                    power-saving mode.
                    Supports SMART auto save timer.
Error logging capability:        (0x01) Error logging supported.
                    General Purpose Logging supported.
Short self-test routine
recommended polling time:    (   2) minutes.
Extended self-test routine
recommended polling time:    ( 185) minutes.
SCT capabilities:          (0x003d) SCT Status supported.
                    SCT Error Recovery Control supported.
                    SCT Feature Control supported.
                    SCT Data Table supported.

SMART Attributes Data Structure revision number: 16
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  1 Raw_Read_Error_Rate     0x000b   100   100   050    Pre-fail  Always       -       0
  2 Throughput_Performance  0x0005   100   100   050    Pre-fail  Offline      -       0
  3 Spin_Up_Time            0x0027   100   100   001    Pre-fail  Always       -       1921
  4 Start_Stop_Count        0x0032   100   100   000    Old_age   Always       -       2846
  5 Reallocated_Sector_Ct   0x0033   100   100   050    Pre-fail  Always       -       269
  7 Seek_Error_Rate         0x000b   100   100   050    Pre-fail  Always       -       0
  8 Seek_Time_Performance   0x0005   100   100   050    Pre-fail  Offline      -       0
  9 Power_On_Hours          0x0032   091   091   000    Old_age   Always       -       3911
 10 Spin_Retry_Count        0x0033   156   100   030    Pre-fail  Always       -       0
 12 Power_Cycle_Count       0x0032   100   100   000    Old_age   Always       -       2838
191 G-Sense_Error_Rate      0x0032   100   100   000    Old_age   Always       -       9
192 Power-Off_Retract_Count 0x0032   100   100   000    Old_age   Always       -       39321611
193 Load_Cycle_Count        0x0032   091   091   000    Old_age   Always       -       96173
194 Temperature_Celsius     0x0022   100   100   000    Old_age   Always       -       32 (Min/Max 11/53)
196 Reallocated_Event_Count 0x0032   100   100   000    Old_age   Always       -       20
197 Current_Pending_Sector  0x0032   100   100   000    Old_age   Always       -       0
198 Offline_Uncorrectable   0x0030   100   100   000    Old_age   Offline      -       0
199 UDMA_CRC_Error_Count    0x0032   200   200   000    Old_age   Always       -       0
220 Disk_Shift              0x0002   100   100   000    Old_age   Always       -       8254
222 Loaded_Hours            0x0032   093   093   000    Old_age   Always       -       2842
223 Load_Retry_Count        0x0032   100   100   000    Old_age   Always       -       0
224 Load_Friction           0x0022   100   100   000    Old_age   Always       -       0
226 Load-in_Time            0x0026   100   100   000    Old_age   Always       -       326
240 Head_Flying_Hours       0x0001   100   100   001    Pre-fail  Offline      -       0

SMART Error Log Version: 1
No Errors Logged

SMART Self-test log structure revision number 1
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Short offline       Completed without error       00%       432         -
# 2  Short offline       Completed without error       00%       430         -
# 3  Short offline       Completed without error       00%       125         -

SMART Selective self-test log data structure revision number 1
 SPAN  MIN_LBA  MAX_LBA  CURRENT_TEST_STATUS
    1        0        0  Not_testing
    2        0        0  Not_testing
    3        0        0  Not_testing
    4        0        0  Not_testing
    5        0        0  Not_testing
Selective self-test flags (0x0):
  After scanning selected spans, do NOT read-scan remainder of disk.
If Selective self-test is pending on power-up, resume after 0 minute delay.

=== START OF OFFLINE IMMEDIATE AND SELF-TEST SECTION ===
Sending command: "Execute SMART Extended self-test routine immediately in off-line mode".
Drive command "Execute SMART Extended self-test routine immediately in off-line mode" successful.
Testing has begun.
Please wait 185 minutes for test to complete.
Test will complete after Sun Jan 17 23:22:49 2021

Use smartctl -X to abort test.

#0. If it's an old HDD, check if it's in a good condition first. HDDs are quite delicate and you may not want to entrust your data to one that's showing signs of aging or nearing failure. Read drive's SMART parameters. This data is collected by the drive itself and will tell you a bit about its condition. In particular look at raw values for reallocated and pending sectors count. Both should ideally be 0. If they are not, this drive shouldn't be trusted. It's fine to keep your games on it (worst case scenario is you'll have to re-download them), but if I were to keep my data there I'd check twice that my backups are working.

#1. Wiping a HDD that you're going to reuse with DBAN is pointless, so save yourself some time and cancel it. As soon as you format the drive, that data won't be referenced by the filesystem anymore and it won't be accessible in normal ways. Data recovery software will be able to read something, but that doesn't matter - it's not like you want to keep its previous content a secret from yourself, you already know what was there.

If you insist on erasing the drive for some reason (eg. sensitive data that should've been encrypted, but wasn't and you don't want people to recover it in case of theft), overwriting the drive with zeros is enough. Data recovery techniques that DBAN and similar solutions protect against were relevant 20+ years ago. Today's technology is just too sophisticated for them to work.

#2.

The HDD is supposed to support a SATA speed of 3Gb/s=375MB/s.

No, it's not. No HDD has ever saturated even a SATA1 link (1.5 Gbps). SATA was built with some headroom that was useful only for port multipliers until SSDs became a thing. 94 MB/s is about right for a laptop HDD.

Whether using an internal SATA port, rather than eSATA, would improve theoretical throughput depends on particular computer's architecture. For example Dell Latitude E5450 which I'm using to type this has a single two-port SATA3 controller. One of the ports is wired for the internal drive and the other one is available as eSATA with full theoretical 6 Gbps link.

#3.

Is it best practice to put the HDD to destroy into the laptop before running DBAN?

As I've already said you probably don't have to DBAN it at all and a regular quick format would work well enough. But if you insist, then no, there's no advantage of installing it inside the laptop when you have eSATA available.

#4.

why does the speed slow down steadily?

The outermost track on a platter contains more data than the innermost one because it's longer. This means that more data is accessible in a single rotation over the outermost track than over the innermost one. Rotational speed is of HDD platters is (roughly) constant, so outermost tracks (corresponding to beginning of the drive) are "faster".

The connection is likely irrelevant. eSATA and SATA are likely to have exactly the same performance for mechanical HDDs.

Your problem is more likely a feature of the spinning platter of mechanical HDDs.

Mechanical HDDs use zoned bit recording with a fixed (ish) physical sector size. What this means is that at the outside of the disk there are more sectors travelling at a higher linear velocity. The angular velocity will be the same as inner sectors, but due to being further from the center spindle their linear velocity is higher for any given drive RPM.

The disk will look something like this:

The net effect is that HDDs are faster at the beginning of the disk, and slower at the inner end tracks of the disk.

An example of what this looks like from a speed perspective can be seen in this Disktester page where you see the disk speed verses the disk position:

Mechanical HDDs very rarely reach the peak speed of a SATA interface. the full SATA speed is only likely to be seen by SSDs and the relatively small drive buffer. Reading from mechanical HDDs very rarely gets above 150MB/s and more commonly 100MB/s for older drives. "Green" or power efficient drives will be worse.

Basically your disk starting at 94MB/s and dropping down to 83MB/s is not abnormal. I'm not surprised at all to see that speed from a 640GB drive.