Can you connect the NAS directly to the PC and write this file to it without issue?
Can you run a scandisk or chkdisk or something?
(I don't have one of these so I don't know how it can be done, but I'd be curious as to the status of the NAS drive.)
---edit---
well I guess if the router is unresponsive probably not the NAS.. well, still worth a check with failed files written now...
I guess I'd put a fan on the router though...