Just get two men to stand on either side.

It's the same problem with a drive like this, or any long term archive, you either store the data unencrypted and rely on physical security, or make sure you store the encryption key and algorithm for the same length of time, in which case you still need the physical security to protect that instead. In both cases you need to make sure you preserve a means to read the data back and details of the format its in so you can actually use it later.

Paper is actually a pretty good way of storing a moderate amount of data long term. Stored correctly it's unlikely to physically degrade, the data is unlikely to suffer bitrot and it can be read back by anything that can make an image in the visible spectrum. That means you can read it, or take a photo and use OCR to convert it into whatever format is current when the data is needed.