I have an external USB drive to type “Seagate Desktop” connected to a Linux system. Although is basically works, the auto standby spindown leads to SCSI errors now and then which at least once led to a backup failing.

I had no success in using hdparm to change the standby parameters, but using sdparm sounds very promising:

~ # sdparm -a /dev/disk/by-id/usb-Seagate_Desktop_2GHK1WEJ
    /dev/disk/by-id/usb-Seagate_Desktop_2GHK1WEJ: Seagate   Desktop           0130
Power condition mode page:
  IDLE        0  [cha: n, def:  0, sav:  0]
  STANDBY     1  [cha: y, def:  1, sav:  1]
  ICT         0  [cha: n, def:  0, sav:  0]
  SCT       9000  [cha: y, def:9000, sav:9000]
~ # sdparm -c STANDBY /dev/disk/by-id/usb-Seagate_Desktop_2GHK1WEJ
    /dev/disk/by-id/usb-Seagate_Desktop_2GHK1WEJ: Seagate   Desktop           0130
~ # sdparm -a /dev/disk/by-id/usb-Seagate_Desktop_2GHK1WEJ
    /dev/disk/by-id/usb-Seagate_Desktop_2GHK1WEJ: Seagate   Desktop           0130
Power condition mode page:
  IDLE        0  [cha: n, def:  0, sav:  0]
  STANDBY     0  [cha: y, def:  1, sav:  1]
  ICT         0  [cha: n, def:  0, sav:  0]
  SCT       4294967286  [cha: y, def:9000, sav:9000]
~ # sdparm -c STANDBY -S /dev/disk/by-id/usb-Seagate_Desktop_2GHK1WEJ
    /dev/disk/by-id/usb-Seagate_Desktop_2GHK1WEJ: Seagate   Desktop           0130
~ # sdparm -a /dev/disk/by-id/usb-Seagate_Desktop_2GHK1WEJ
    /dev/disk/by-id/usb-Seagate_Desktop_2GHK1WEJ: Seagate   Desktop           0130
Power condition mode page:
  IDLE        0  [cha: n, def:  0, sav:  0]
  STANDBY     0  [cha: y, def:  1, sav:  0]
  ICT         0  [cha: n, def:  0, sav:  0]
  SCT       4294967286  [cha: y, def:9000, sav:4294967286]

There are a little bit more elaborated solutions to be found here

Linux disk USB