Is it the norm for makers of DVRs to advertise the feature of being able to add on storage arrays, and then, without stating such up front, make the compatibility a proprietary interface, so you can only use thier devices?
Most makers offer their own ready made add on storage, which has been configured, and tested throughly to work with their equipment. Most of the expansion I've seen has been SCSI (and now some USB and SATA is starting to appear), which you could probably build a custom add on for. However, the manufacturer would advice against it, because they cannot test every configuration possible, and they won't warrenty any damage that could be caused by some off the wall setup. It's most likely possible (there are only so many interfaces that exist for hard drives), the companies 1: don't want to risk the customer screwing something up by 2: offering a product that may be a bit more than you could buy yourself at newegg, but it has been engineered and tested to work reliably with the product.
There are many ways to use storage for video. You are not mentioning what type of DVR you are using, but it is unlikely that a simple DVR manufacturer would R&D his own complete storage solution. Have you looked at places like
for some direct attached solutions? You may find the clone of what you think is proprietary gear. Unfortunately many of these solutions are not available to anyone without special training or other arrangements with the manufacturer. If you are talking about an iSCSI solution, instead of direct attached storage, there is a problem there with many units due to the number of LUNs available. Typically every video inputs from an IP address (like an IP camera or video encoder) gets its own LUN. Many manufacturers create iSCSI arrays geared to work with a single server or smaller group of servers. Therefore you might find an iSCSI device with enough total storage capacity to meet your retention time needs, but lacking the LUN capacity to handle the number of inputs you plan to deploy. However, if you keep looking I think you might find the actual storage manufacturer. Getting a unit like that configured and working and keeping it working without factory tech support might make you wish you hadn't found it. .