External USB drive seems to slow down, ideas?

[zambz]

Hey there guys, hope you're doing great.

I have several external mechanical drives I use for archiving all my work once it's complete. I always have two copies of everything across two drives. These drives are formatted with Mac OS Extended (Journaled) and have been excluded from Spotlight search.

The drives I'm working with at present are 3 TB Western Digital Essential drives which usually have a read / write speed of around 120 MB/s.

The disks both currently have 2 TB used and 1.1 TB available.

When performing a disk speed test in Blackmagic Disk Speed Test, one of the drives yields around 35 MB/s and the other around 65 MB/s read and write speeds.

I have run First Aid and also a Full Defrag in iDefrag (which took many days) on the slower drive just to see if it will make a difference. I've also ensured the hard disk is connected directly to my MacBook Pro and have tried different cables too. But none of these solutions have worked and I'm still left with very slow speeds.

I also downloaded DriveDx and ran a S.M.A.R.T. test on the drive, but everything came up clear.

Interestingly I experienced a very similar issue with an external USB Seagate drive I have. Wiping the entire disk brought the performance up again and it has stayed fast since, but I never got to the bottom of why that had gotten so slow too.

Right now, I'm transferring the data from my slow 3 TB drive to another drive so I can wipe it and start over and here are the speeds I'm seeing with iostat:

fots ~ $ diskutil list /dev/disk3
/dev/disk3 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *3.0 TB     disk3
   1:                        EFI EFI                     314.6 MB   disk3s1
   2:                  Apple_HFS Sample Libraries 2      3.0 TB     disk3s2
fots ~ $ iostat disk3 1
              disk3       cpu    load average
    KB/t  tps  MB/s  us sy id   1m   5m   15m
 1202.92   11 12.47   2  2 97  1.37 1.50 1.50
  128.00  319 39.91   1  3 96  1.37 1.50 1.50
  128.00  317 39.67   1  2 96  1.34 1.49 1.50
  128.00  318 39.80   1  3 96  1.34 1.49 1.50
  128.00  317 39.68   2  3 95  1.34 1.49 1.50
  128.00  319 39.92   2  3 96  1.34 1.49 1.50
  128.00  318 39.74   1  2 96  1.34 1.49 1.50
^C

I've tried researching this far and wide and just can't find anything else useful.

Any help is greatly appreciated! Thanks heaps guys

 

[BrianBaughn]

What exact model Mac and what version of macOS?

What kind of read/write speed are you getting on the "another drive" you mentioned?

Report back with the performance after reformatting.

My first guess, though, is bad blocks on the drives. Does DriveDX do a complete scan for bad blocks?

My theory on the improvement after reformatting, without doing so much as a click of research, is that, even with bad blocks, drives perform better when they're freshly written. When data starts getting moved around the bad blocks have more of an effect on operation. End of theory.