This sounds like samba server on DD-WRT is not reporting disk size or available space properly. MAC OSX Finder decidedecides it can't try copying files while Ubuntu tries it anyway. The problem sounds to be coming from DD-WRT samba server. Any idea how to solve this? I did look extensively on DD-WRT forum and no luck.
I found a YouTube Video where it shows an Asus router (I use TP-Link TL-WDR3600) with DD-WRT V24Sp2 like mine having the same drive size (2TB) shared and we see the proper available space displayed in Finder. Nothing special was done during configuration. Other than using a different router, his disk has HPFS/NTFS format instead of EXT3.