Have you setup CISF/SMB shares with user/password and given write permission?
Have you activated the FTP server and again setup an account with user/password?
How are you trying to copy the files from your computer to the NAS?
What version of windows are you using (there have been reported problems with win10 CIFS/SMB)?
Sorry to ask these questions but their answers do give us a fighting chance of helping you with your problem.
For instance, if you have setup CISF/SMB Shares then you should be able to drag and drop any size file from your computer to the NAS using windows explorer, with the only problem being win10 in some cases.
Likewise, using the NAS FTP server and FileZilla FTP client you should be able to, again, move large files to the NAS (we regularly move 40 GB files to several of our NAS boxes once a week using FTP without any problems).