Network connected Tivo's on the same account can transfer and stream content with each other. You can also transfer content to store and watch on a pc and even transfer it back to one of your Tivo's. There is software available that will allow pc stored content to be converted to other formats that can be watched on mobile devices or on someone else's pc (which can be watched on Tivos attached to a different account). And there is software available that allows streaming of pc stored content to your Tivo's. The only content that can't be transferred are cable recordings that are flagged "copy once" (like HBO). Although you can still stream it between two units. Content recorded on a Tivo is your's to keep and not the providers to take away.
And TiVo doesn't really like this, but they have the legacy of open source and has a legacy BEFORE the content providers and owners became more restrictive. This is why no subsequent DVR (and, of course, RePlay TV with similar features was released at the same time as TiVo--along with the DishPlayer 7000 with software by Microsoft--in those content owner ignorant days) has ever had a transfer to PC feature. The content owners have come to DESPISE this. Also, because of the open source, there are all sorts of published work arounds on the internet for sorts of little extras for all boxes up to the Series 4. The Series 4 comes with a few more restrictions in this regard, but not entirely locked down, but being locked down is TiVo's goal, and the TiVo forums grumble about it. And most TiVo users find the officially sanctioned TiVo Desktop to restrictive and use alternatives from the net, and now TiVo wants to charge for any TiVo Desktop.
A TiVo external HDD is "married" to that particular DVR forever and can be used only to increase recording capacity, functioning like a Dish 211K, not for archiving, an inferior solution in my IMHO, and I am a TiVo owner like the TiVo product. When that HDD dies, so do ALL of your recordings, even those that may be on the internal HDD. Disconnect the external HDD and changing it, than the same result: All recordings lost, even those on the internal HDD as some recordings are actually spanned between the internal and external HDD.
However, the key for the external HDD's on Dish is tied to the account, and when the use of HDD's is at the conditions that content providers and owners will allow. Dish and other MVPD's or TiVo don't necessarily care what you do with the content (except for when the content people pressure them), but ALL restrictions for any of these devices is all imposed by the content owners and providers. It may have been the only way Dish could get them to agree to an external HDD feature without being sued, which means a court will halt production and sale of such disputed device or service until resolved, many years later. So, it is all about getting everyone to agree to a scheme and move forward with no lawsuits. Not consumer friendly at all.