Backup failed with:
Code:
erlangen:~ # journalctl --since 07:00 --until 07:10 -u backup-home-WD25.service   
-- Logs begin at Fri 2021-06-04 21:27:14 CEST, end at Wed 2021-06-16 15:14:22 CEST. -- 
Jun 16 07:03:18 erlangen systemd[1]: Starting Backup /home to USB disk... 
Jun 16 07:03:22 erlangen rsync[13087]: rsync: [generator] recv_generator: failed to stat "/WD25/backup/home/Albums/Bilder/2015/2015-02-12/20150216075315-77010-map.html": Input/output error (5) 
Jun 16 07:03:22 erlangen rsync[13087]: rsync: [generator] recv_generator: failed to stat "/WD25/backup/home/Albums/Bilder/2015/2015-03-10/20150316022150-04180-map.html": Input/output error (5) 
Jun 16 07:03:29 erlangen rsync[13087]: rsync: [generator] recv_generator: failed to stat "/WD25/backup/home/Albums/Bilder/2017/2017-06-16/20170617124057-07168-map.html": Input/output error (5) 
Jun 16 07:03:34 erlangen rsync[13087]: rsync: [generator] recv_generator: failed to stat "/WD25/backup/home/Albums/Bilder/2018/2018-05-30/.jalbum/p1230368.mp4.info": Input/output error (5) 
Jun 16 07:03:39 erlangen rsync[13087]: rsync: [generator] recv_generator: failed to stat "/WD25/backup/home/Albums/Bilder/2019/2019-09-04/p1310878.jpg": Input/output error (5) 
Jun 16 07:04:27 erlangen rsync[13087]: rsync: [generator] recv_generator: failed to stat "/WD25/backup/home/Albums/jAlbums/Bilder/2009/2009-07-14/thumbs/imgp7089.jpg": Input/output error (5) 
Jun 16 07:04:32 erlangen rsync[13087]: rsync: [generator] recv_generator: failed to stat "/WD25/backup/home/Albums/jAlbums/Bilder/2010/2010-05-19/slides/imgp9397.jpg": Input/output error (5) 
Jun 16 07:04:34 erlangen rsync[13087]: rsync: [generator] recv_generator: failed to stat "/WD25/backup/home/Albums/jAlbums/Bilder/2010/2010-08-09/thumbs/imgp0064.jpg": Input/output error (5) 
Jun 16 07:04:37 erlangen rsync[13087]: rsync: [generator] recv_generator: failed to stat "/WD25/backup/home/Albums/jAlbums/Bilder/2014/2014-02-01/thumbs/p1010670.jpg": Input/output error (5) 
Jun 16 07:04:38 erlangen rsync[13087]: rsync: [generator] recv_generator: failed to stat "/WD25/backup/home/Albums/jAlbums/Bilder/2014/2014-03-26/thumbs/p1020445.jpg": Input/output error (5) 
Jun 16 07:04:39 erlangen rsync[13087]: rsync: [generator] recv_generator: failed to stat "/WD25/backup/home/Albums/jAlbums/Bilder/2014/2014-05-17/slides/p1030429.jpg": Input/output error (5) 
Jun 16 07:04:58 erlangen rsync[13086]: rsync error: received SIGINT, SIGTERM, or SIGHUP (code 20) at rsync.c(701) [sender=3.2.3] 
Jun 16 07:04:58 erlangen systemd[1]: backup-home-WD25.service: Main process exited, code=exited, status=20/n/a
Jun 16 07:04:58 erlangen rsync[13087]: rsync error: received SIGUSR1 (code 19) at main.c(1608) [generator=3.2.3] 
Jun 16 07:04:58 erlangen systemd[1]: backup-home-WD25.service: Failed with result 'exit-code'.
Jun 16 07:04:58 erlangen systemd[1]: Stopped Backup /home to USB disk. 
Jun 16 07:04:58 erlangen systemd[1]: backup-home-WD25.service: Consumed 17.297s CPU time.
erlangen:~ #
Unmounted and ran fsck on 2TB ext4. Got prompted for continuing after read error in directory inode. When prompted for overwriting I did so. Fsck would hang and even 'kill -9' wouldn't work. Thus I unplugged the USB drive and plugged it in again. Another fsck reported a clean file system. I started a long selftest (400 minutes):
Code:
erlangen:~ # smartctl -l selftest /dev/sdd            
smartctl 7.2 2020-12-30 r5155 [x86_64-linux-5.12.9-1-default] (SUSE RPM) 
Copyright (C) 2002-20, Bruce Allen, Christian Franke, www.smartmontools.org 

=== START OF READ SMART DATA SECTION === 
SMART Self-test log structure revision number 1 
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error 
# 1  Extended offline    Completed without error       00%       899         - 
# 2  Extended offline    Completed without error       00%         8         - 

erlangen:~ #
Restarted backup service and got a nice message:
Code:
erlangen:~ # journalctl --since 13:00 -u backup-home-WD25.service                 
-- Logs begin at Fri 2021-06-04 21:27:14 CEST, end at Wed 2021-06-16 15:24:47 CEST. -- 
Jun 16 15:07:31 erlangen systemd[1]: Starting Backup /home to USB disk... 
Jun 16 15:12:55 erlangen systemd[1]: backup-home-WD25.service: Succeeded. 
Jun 16 15:12:55 erlangen systemd[1]: Finished Backup /home to USB disk. 
Jun 16 15:12:55 erlangen systemd[1]: backup-home-WD25.service: Consumed 42.747s CPU time.
erlangen:~ #
Smart attributes are what they should be:
Code:
erlangen:~ # smartctl -A /dev/sdd                                 
smartctl 7.2 2020-12-30 r5155 [x86_64-linux-5.12.9-1-default] (SUSE RPM) 
Copyright (C) 2002-20, Bruce Allen, Christian Franke, www.smartmontools.org 

=== START OF READ SMART DATA SECTION === 
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     0x002f   200   200   051    Pre-fail  Always       -       0 
  3 Spin_Up_Time            0x0027   208   200   021    Pre-fail  Always       -       4600 
  4 Start_Stop_Count        0x0032   100   100   000    Old_age   Always       -       598 
  5 Reallocated_Sector_Ct   0x0033   200   200   140    Pre-fail  Always       -       0 
  7 Seek_Error_Rate         0x002e   200   200   000    Old_age   Always       -       0 
  9 Power_On_Hours          0x0032   099   099   000    Old_age   Always       -       899 
 10 Spin_Retry_Count        0x0032   100   100   000    Old_age   Always       -       0 
 11 Calibration_Retry_Count 0x0032   100   100   000    Old_age   Always       -       0 
 12 Power_Cycle_Count       0x0032   100   100   000    Old_age   Always       -       302 
192 Power-Off_Retract_Count 0x0032   200   200   000    Old_age   Always       -       258 
193 Load_Cycle_Count        0x0032   200   200   000    Old_age   Always       -       2471 
194 Temperature_Celsius     0x0022   113   101   000    Old_age   Always       -       39 
196 Reallocated_Event_Count 0x0032   200   200   000    Old_age   Always       -       0 
197 Current_Pending_Sector  0x0032   200   200   000    Old_age   Always       -       0 
198 Offline_Uncorrectable   0x0030   100   253   000    Old_age   Offline      -       0 
199 UDMA_CRC_Error_Count    0x0032   200   200   000    Old_age   Always       -       0 
200 Multi_Zone_Error_Rate   0x0008   200   200   000    Old_age   Offline      -       0 

erlangen:~ #
Note the 899 Power_On_Hours. An error can occur at any age of the drive.