– IX4-300D with SSH access: SSH Guide
– Custom startup script: Guide Here
– Follow guide at own risk, I am not responsible for any damage!

SSH to your NAS, go to your OPT folder and edit init-opt.sh:

nano init-opt.sh

Now paste following lines somewhere at the bottom of the script :

#set remove permissions for all shares to 777 (fixes unremovable downloads)
chmod -R 777 mnt/pools/A/A0/

Save the script (CTRL+O) and restart your NAS, when fully booted, the NAS will (reset) all file permissions and all folders&files can now be deleted again

!The process may take some time, 8Tb would take about 10 minutes to complete so when rebooting the first time, leave some time for the process to complete.