Can I store my stuff in any container in the Dawnguard castle?

After you finish the campaign on the Vampire Side you can use all the containers in Harkons room.

Didn't he ask about Fort Dawnguard?

From what I know, Fort Dawnguard isn't a safe location to store your stuff.

Not safely. The only safe storage containers in Fort Dawnguard are the cupboards marked as "Wardrobe." The barrels, sacks, and even the chests are unsafe. Tested myself.

Lol, yes he did I answered a bit quick.

I haven't found any safe storage in FD. I've been testing all chests, wardrobes, tables, and dressers with ingots, and even after a short amount of time, stuff I put in there is gone.

Ugh, that stinks, since I had planned on doing the Dawnguard side next.

Well, there's always Honeyside.

I never found any safe storage in the Fort either, but at least the mannequin never lost the armor, my character returned after a very long absence to find it had multiplied, the mannequin had 3 of every piece of Dawnguard gear it had been wearing. The weapon rack was still bugged in our room.

I simply don't understand why Bethesda could not code the mannequins/weapons racks/bookshelves properly in the first place, or establish safe storage in areas like Fort Dawnguard. It really bugs the hell out of me this stuff happens. :\

The chest upstairs in whats his names room (the Dawnguard leader) may try to trick you as well. It had been holding all my stuff for the entire DG campaign. Then I loaded Heathfire and came back to an empty chest. Ugh....so many pieces of unimproved armor that I was going to bang out some smithing skill on....gone.

Lesson learned. Apparently there is no safe place to store stuff in Fort Dawnguard.

I've used the wardrobes up in Isran's room without problems. (Upstairs next to the torture chamber. Cozy.) There's also an endtable-type piece of furniture in the barracks room that I use for alchemy and smithing materials. These appear after some upgrades, I don't remember exactly when but it's early. I don't trust the chests, and I never use a sack or barrel that's outside an owned house.

Hopefully after the PC people get into this and can check out the place in the CK, we'll have some better information.