I'm going to get into trouble for this but the Blackmagic device uses commodity HDDs or SSDs, is tiny and cheap. If you want to record companion video as uncompressed (a lot of folk do still prefer uncompressed) then it's fabulous. If you'd rather record to ProRes, then I'd go for the Ninja from Atomos. Way smaller and cheaper than the other devices (aside from the BMD one), has an LCD screen and, once again, uses commodity drives. You can even use it as a viewfinder with their cheap hoods and diopters. There are a number of devices from AJA, Sound Devices etc. but all larger, heavier and more expensive. I know for a fact that both BMD and Atomos are working with Red right now to ensure their devices will work nicely with the Red output - ie: will record/pause with the camera and will record the file names used in-camera for easy link-back in post. This means you never have to actually drive the recorder and you'll always have the option of going back to the R3Ds in post if you need to.