For the most part CM answers questions, and will probably answer your question about the maximum EHD size. You can call them, or do like I do and email them so that I have their reply in writing! I will be pleasantly surprised if they say anything larger than 3TB (because they haven’t tested many drives, and only guarantee the models they sell), and the largest they sell is 3TB. Before contacting them, read their Q&A page on “Why is my DVR+ External Hard Drive not working correctly?”, as it’s fairly informative, although they don’t mention the maximum size:... If I call the people at CM and ask them what is the maximum EHD size the machine will recognize will they answer my question or is there some reason why they want to keep it a secret.
The main Q&A page is at http://support.channelmaster.com/hc/en-us/categories/200031460-DVR-
FYI, the question they refused to answer, and later refused to acknowledge on facebook, was the issue with the 2TB limitation on disks larger than 2TB, but they quietly fixed it in 123/124R.