"Sundered" is a term for early Slowboat space colonies which successfully survived colonisation of their target worlds and did not become claim-jumped Forsaken, only to find with the discovery of FTL Travel, they were disconnected from the rest of Humanity.
Many modern Sundered colonies have joined the Forsaken. Others have become so-called "Neo-Barbarians", taking to isolationism from the Jump Network and occasional - but vicious - piracy.
To understand the problem facing Sundered worlds, one must recall that only a small fraction of all systems are connected via the jump network. A fair number of colonies already settled and colony missions in-flight and out-of-contact at the time of the discovery of FTL and the ensuing Nano-Plague were on or travelling to systems not connected to the jump network. This did not spare them from the nano-plague, but did greatly hinder their recovery, as recontact through the SPEC drive was not possible for several hundred years.
Many of these colonies never recovered from the plague. A fair fraction of them were completely lost, including such formidable notables as the Great Mormon Mission. Some worlds bear evidence that the colonists attempted to fight the nano-plague directly: their civilizations were razed to the ground when it fought back, although exactly how remains unclear.
Those living on more innately human habitable worlds met a variety of fates, each living through the nano-plague in their own ways. Of those that survived the nano-plague, many did not survive as anything resembling their initial civilizations.
SPEC and Recontact
SPEC travel was not invented for several hundred years after the development of the jump drive and has remained inconvenient (albeit decreasingly so) for interstellar travel through into the modern era. Recontact through SPEC of isolated Sundered worlds offered help to those rare few still combating the nano-plague. Some groups, lucky in a morbid way, produced so few survivors by the time exploratory SPEC craft arrived that rescue operations were affordable, and resettlement often occurred in Forsaken space. Other, larger groups of survivors could not or would not be efficiently transported to Jump-capable systems; despite SPEC-drive making transport possible, these groups remain very isolated from the rest of space and Humanity, both physically and culturally. They comprise what are now called the "Neo-Barbarians".
It is, of course, a bit of a misnomer to call them Barbarians, as they are not so much barabaric or savage as merely culturally and economically divergent from the rest of humanity (far more so than even the Forsaken). Of course, taking the original meaning of barbarian, it is perhaps an accurate moniker.
Spread of Technology
Those who continued on in their existence outside of the jump network were not privy to centuries of advancements or cross-cultural pollination. Many of these groups were of particular interest for Luddite recruiters, seeking both isolated bases of operation and untainted samples of humanity. Thus, through Luddites and the odd profiteer, robber baron, or fool, did the isolated and technologically backwards Sundered gain access to more modern craft and resources.
This gave rise, among those Sundered civilizations that had become more aggressive during their isolated survival struggle, to the use of their newfound toys to prey upon their neighbors for anything they could not produce or otherwise acquire.
It is interesting to note that, alone among the human groups, those more prosperous among the Sundered continue to colonize systems off the jump network. However, due to the impracticality of scaling SPEC travel to trading routes beyond astronomically adjacent systems, the overhead of even that, and the impossibility of swift communication, there are no empires as such. Each system or at most cluster of systems is given to its own governance, and development is greatly slowed due to the trickle of interaction with other groups. The particular interest the Luddites have placed in a subset of the Sundered has led to significant tensions between those Sundered groups receptive to the Luddites and all other Sundered polities.