Greetings Adrian,
it’d be nice of you if you could e.g. fill out the template I personally provided here. It will save you time summarizing your system specs and help us help you better.
Additionally, provide the exact commands you are issuing and please elaborate on what kind of data you try to copy from where to where. (When filling out the template, the commands requested will display the storage media you use, so you can precisely point to which are being used how.)
It would also be helpful if you could run
apt update; apt install -y smartmontools
for i in {a..d}; do echo "Disk sd$i" $SN $MD; smartctl -a /dev/sd$i; done
which will display the SMART values for your drives.