The barrels and chests are not safe as i understand.

AFAIK, there's no place in Fort Dawnguard that's safe for storage, including the floor. The whole cell resets after a certain period of time as long as you are away from it.

If you're planning to join the DG, it's best to acquire Honeyside in Riften before you start the quests if you want safe storage.

Ahh ok, bugger. Ah well, i have Lakeview manor.

You could always make a small house in the DG valley behind the walls using the CK.

I believe that the vampire castle is almost fully safe though - don't know why they did it with one, and not the other.

Assuming he's a PC player.

at OP, you can check by dropping a useless item down and then waiting somewhere for 10days. Assuming the default reset time is still 10. In Skyrim, the outside world is a gigantic storage place. You can drop whatever you want down, and it'll still be there despite resets. Be aware though that NPCs can pick these up.

Fort Dawnguard indeed has a 10 day respawn time. (Source: http://www.uesp.net/wiki/Skyrim:Fort_Dawnguard )

I never tried storing all my armor on a mannequin, but that mannequin and the weapon rack were the only things in that fort that never lost what I gave them, mind you when Aussie went back ages later, the mannequin had three of everything she had put on it, no idea how that happened.

Even if the floor is safe, which it probably is, I would never leave stuff lying around on the floor for so many NPC's to claim.