The somewhat tricky part when removing the harddrive, is to get that metal bracket out, that the harddrive is mounted in, that attaches to the bottom of the inside of the rack unit.
I forgot the size, but you need a ratchet and a fairly long extension (or two shorter ones connected together, like I do), and a small socket... it might have been an odd size, like 7 mm or so, but was somewhere between 7 to 10 mm.
With this being a device designed in the US, it might be an "imperial" socket size, though. I just usually look for one that fits, and quickly forget again what the size was, haha. But if you have a set of small-ish metric and imperial sockets (like most people do), you should have what's necessary.
There's 4 nuts on posts attached to the bottom of the D8B rack unit that you have to remove. Then you can take the whole mounting bracket out, and then use a phillips screwdriver to remove the harddrive, as David said.
(Of course you first need to open the D8B rack unit itself... that's also a phillips screwdriver all around).
What's a bit tricky, is getting the mounting bracket back in, because it's hard to reach down to those posts with your hands. I usually use a chop-stick and stick the nut on there tight, then put it over the post, and push it off of it... then keep holding it in place with the chopstick while I twist the nut with some other long thing until it catches the thread.
...or you could use some play-doh or chewing gum or something similar and stick it in the socket to hold the nut while you're threading it onto that post... but I hate that sort of mess in my tools, and got pretty good at the chopstick method, haha
Anyway... there's no special tools needed. It's just a tight space to work in, so you need to find a way to reach those nuts on the harddrive mounting bracket on the bottom of the inside of the D8B rack unit.
Best of luck!