Some amendment to my current situation.
In the previous posting, I mentioned about backup successful after switching off "auto detect settings". But the fact is, I successfully backup to NAS not because I switched off this setting, but I modify the maximum size of backup-to-disk file to allow 100GB.
I made this change because I observed the backup fails everytime when current file is filled up. So, I enlarge the maximum size so th system does not require to open more than 1 backup file.
The backup finishes at 5 hours now, though still slow comparing to my desktop, it is more or less acceptable. This is the problem of the NAS being run on software RAID.