Floor safes will not keep out settlers.

[rookie]

I read online that if you put items in floor safes settlers wouldn't access them. This is false information.

I just happened to be at my floor safe where I stored all my ammo and fusion cores when a provisioner was attacked by 1 raider. I witnessed 6 settlers rush to the work bench and floor safe, which was next the work bench, and grab items.

These settlers were equipped with assault rifles,combat rifles and laser rifles before the attack.
It made no difference as they took more assault rifles, long gamma rifles and 100's of rounds of ammo.

I moved all weapons,ammo, fusion cores and power armor to a unpopulated settlement. But for a provisioner I hope these items stay untouched.

[DouglasGrave]

The mannequin "Armour Racks" from the Contraptions Workshop DLC are your easiest option for safe storage. They're not limited to just storing armour, and because they work like frozen NPCs, other NPCs don't treat them as a lootable container.

Crafting workbenches are safe if they're out in the wilderness, but not if they're in one of your settlements (because in settlements they're connected to the main workbench, which is vulnerable).

It's not exactly a settlement, but Home Plate in Diamond City is a location to safely store things, if you need it. If you're not playing on Survival mode, just keeping all the ammunition yourself is viable.

[=EGC= kansasterry]

All the usual containers can be looted by settlers and npcs that happen to be in the settlement when an attack happens. This includes attacking npcs.

There is nothing that can be done about this as the teleport fix for navmesh issues will teleport an npc through any wall or locked door to reach their ai packages target if it times out trying to get to it.

The armor racks mentioned in the above post are safe because the game sees them as a display rather than a container.

[mikk011]

My solution is to store restricted items in the Root Celler in Sanctuary. Settlers don't go there. Many other posters just store their items in a non-populated settlement.

[WillieSea]

I store all my stuff in the Levelers Bunker, automatically, which is just up the hill from Sanctuary. And I can automatically fill up my stimpacks, fusion cores, selected ammo or scrap from those supplies. All at the execution of a holotape from anywhere in the commonwealth.

I am actually working on updating that mod now.

[Crystal Sharrd]

I keep a settlement just for myself, and that's where I store all my loot.