Here I shoot some pictures of my Board. I think it is the same as you said,
I added on other site too: http://iomega.nas-central.org/wiki/Category:EZ_Media
On 3rd image there is a populated 4-pin connector labeled "CN4" that is seria-ttl, it is at bottom left.
The pins are from left to right tx gnd rx, the 4th one is not needed.
I had some time now to test Debian, it is difficult to install it ?