My Cloud EX4100 USB Backup Failed

Hello, I have several problems with backuping USB drives to my EX4100 NAS,
while backuping USB to NAS using 4TB Lacie 2,5" USB drive and WD Elements Portable - 4TB 2,5" drive. (Both disks are containg backups files from my computers) after some time the backup proces stopped just NAS is saying “Backup failed, check system settings”
How can I fix this issue, I d like to know where is the problem,
is there some log where I can find out which files or path is making this issue?
What is the maximum folder path lenght?

While backing up similar data from smaller drives - WD MYpassport 2TB everything is going ok…

Thanks for advices