TCP retransmits will not hurt file integrity, so you can rest easy on that one. They will slow transfer speeds, but occasional retransmits are probably something you can live with.
You might look at the switch stats - keeping in mind that they generally report from the switch perspective (tx on the switch is rx on the NAS).
One possible cause is buffer overrun in the switch - since you were pushing a lot of data when you got the error. Look to see if 802.3x flow control is enabled on the switch. Also, if there are QOS settings, you can possibly prioritize traffic to/from the NAS.
Another possibility (generally rare these days) is that the switch thinks the link is half-duplex, but the NAS or PC thinks it is full duplex. If that is the case, you can usually manually override the setting on the switch.