Readynas rn212 no network after lightning strike

Yes, it likely fried the Ethernet transceiver.  It's not a "card", so can't be replaced.  No, you cannot use a USB to Ethernet adapter or any other method of accessing the NAS once the Ethernet is dead.  Given they share a lot of circuitry, it's likely both ports are fried, but did you try them both?

This is but one reason a backup is necessary when using a NAS -- catastrophic things can happen to make your data inaccessible.  Of course, were you doing so to another NAS, it may well have also been fried.

Given the nature of your question, I'm assuming you have no backup.  The simplest way to recover the data is to replace the NAS with another ReadyNAS.  Any ReadyNAS running OS6 will work, including legacy models converted to OS6.  You will, of course, have to purchase used.  And be aware that your caddies will not work with a legacy unit, so don't buy one that comes without caddies.

There are other options using a Linux system or (expensive) RAID recovery software on a PC.  If you want more info on these options, post back that you do.

FYI, it is unlikely that the damage was due to a "power surge".  The wall wart power supplies of routers usually protect from that.  More likely, it suffered from a ground plane voltage difference, causing a current to flow down one of the LAN cables from a device on one ground to a device on an independent ground.  If your internet is provided by cable, check to see if the cable and your main power are tied to the same ground point.  They often are not, which is very bad.  You can reduce the chances of a similar event in the future by tying the two ground points together with a copper cable (I recommend 0 gauge, but anything is better than nothing). 

If you don't have cable internet, or even if you do, look to see if any other device is tied to a separate ground point.  An external TV antenna is one possibility.