I use the WD My Passport Wireless. Yes, you can transfer raw files from an SD card on to it, and the software app will allow you to view the copied SD folders but it does NOT show raw previews. It does allow other apps that can handle RAW files to register and you can use Open In but you have to do them one at a time, blindly, by filename. Unfortunately the easiest workaround is to shoot raw + jpeg. Then at least you can see the jpeg thumbnails directly, open them to view larger, and then use Open In on the corresponding raw file you want to work with. Unfortunately, as of this writing, none of the apps I would really like to use for raw have registered for Open In, e.g. Lightroom Mobile, PS Express or any other Adobe PS related apps. IIRC Snapseed and one or both of the Filterstorm apps are available and work well (I use Canon CR2 raw files). I am hoping this situation changes soon, especially with the announcement that the iPad Pro will soon get better native raw file handling.
On my Passport Wireless, the copy of one fairly full 32GB card takes a good half an hour and consumes more than 50% battery if started from full. So make sure you have auxiliary power for it. There are newer versions (My Passport Wireless Pro, I think it's called) that hopefully have improved on both of these things. If you want to later import the copied SD directories to a computer with, say, Lightroom, that works well because the USB connection is USB 3.0 and of course, when connected this way, it gets power from the computer.