Hi all,
I was caught out by the fact that there were no 19" rack mounts in with the RX4100.
Post http://community.wd.com/t5/WD-Sentinel-Products/Rack-Mount/m-p/584201/highlight/true#M1955 also talks about them not being there, but I could not reply to it.
The rack mounts are extra, and you need the slide rail kit, see http://store.westerndigital.com/store/wdus/en_US/DisplayAccesoryProductDetailsPage/ThemeID.30153100/productID.289203100/categoryId.58949100
I hope that this is useful.
Regards,
Tony