Cron DD backup doesn't work

crontab -l
# Edit this file to introduce tasks to be run by cron.
#
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
#
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').
#
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
#
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
#
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
#
# For more information see the manual pages of crontab(5) and cron(8)
#
# m h  dom mon dow   command
30 * * * * /usr/bin/restic -r /mnt/dietpi_userdata/restic/dietpi backup / -x --compression max --pack-size 128  -p /mnt/dietpi_userdata/haslo.txt
#* * * * 1 /usr/bin/restic -r /mnt/dietpi_userdata/restic/dietpi prune --max-unused 0 --compression max --pack-size 128  -p /mnt/dietpi_userdata/haslo.txt
41 * * * * /usr/bin/dd if=/dev/mmcblk0 bs=1M| gzip -c -1 > /mnt/dietpi_userdata/`date +%F`.img.gz

/usr/bin/dd if=/dev/mmcblk0 bs=1M| gzip -c -1 > /mnt/dietpi_userdata/date +%F.img.gz
manually started it works

did you check the cron log?

journalctl -u cron.service
Oct 16 20:40:02 DietPi CRON[15332]: pam_unix(cron:session): session closed for user dietpi-ddns
Oct 16 20:41:01 DietPi CRON[15383]: pam_unix(cron:session): session opened for user root(uid=0) by (uid=0)
Oct 16 20:41:01 DietPi CRON[15384]: (root) CMD (/usr/bin/dd if=/dev/mmcblk0 bs=1M| gzip -c -1 > /mnt/2tb/`date +)
Oct 16 20:41:01 DietPi CRON[15383]: (CRON) info (No MTA installed, discarding output)
Oct 16 20:41:01 DietPi CRON[15383]: pam_unix(cron:session): session closed for user root
Oct 16 20:50:01 DietPi CRON[15644]: pam_unix(cron:session): session opened for user dietpi-ddns(uid=998) by (uid=0)
Oct 16 20:50:01 DietPi CRON[15645]: (dietpi-ddns) CMD (/var/lib/dietpi/dietpi-ddns/update.sh)
Oct 16 20:50:04 DietPi CRON[15644]: pam_unix(cron:session): session closed for user dietpi-ddns

/mnt/2tb/ and /mnt/dietpi_userdata/ are the same HDD

Your command it not interpreted correctly.

/usr/bin/dd if=/dev/mmcblk0 bs=1M| gzip -c -1 > /mnt/2tb/`date +

As an example, it should look like following

CURRENT_TIME=date +%Y.%m.%d_%H:%M:%S.%3N
* * * * * /bin/echo "complex variable test! $(${CURRENT_TIME})__test!" >> "/tmp/$(${CURRENT_TIME}).test.log"

Created simple script

#!/bin/sh
/usr/bin/dd if=/dev/mmcblk0 conv=sync,noerror | gzip -c -1 > /mnt/2tb/$(date +%F).img.gz

which is started from cron.

Thank You for help.