It has been a while since this idea was last discussed. I'm not sure if this is the most recent thread, but here's one thread on it.

Similar to the recent comments, I think the usefulness of the idea depends on whether the nature of the disaster and whether your storage location would be inside the affected area. E.g. if a hurricane suddenly turned and was heading your way and all the roads are jammed with people trying to evacuate, then any storage facility within bicycling distance would definitely be in the affected area. The staff there will be locking it down and getting the heck out of there along with everyone else. Even self-serve locations where you use a key or key card to open a gate and then drive up to your storage "shed" will be locked down to prevent theft while everyone is gone, assuming that power is even available to power the lock and gate.

So, I think prepositioning has merit if you can reasonably be sure that you can access the supplies. A hole in the ground will always be accessible, but vulnerable to theft and damage from being buried. A friends house will generally be safe, but you may not necessarily be able to count on access, particularly if you're delayed and your friend wants to evacuate. Anyway, it's a good idea, but there are a lot of practical obstacles to overcome to make it work reliably.