I Hadn't Thought of That!

When you delete a file it doesn't go away, the data is still on the disk, but the the reference to where that data is stored is taken away. It also isn't overwritten because the drive simply maps another portion of the drive to fill the gap. This technique is used so that the information is recoverable using forensic tools, and the extra space serves as a backup in case an error occurs in that block or if you need to shift data from one place to another. This is interesting in the case of the Westworld hosts brains because it would seem that with a nearly infinite amount of storage space, the data that has been "wiped" is never actually overwritten, but the reference to it is taken away. This could explain a lot of things that are going on with the host's memories, i.e. not being able to determine the metadata associated with the memory such as time and place. Also, the "reverie" would seem to work as a sort of forensic tool that allows the host to cross reference data only by association because the direct reference is lost, but that data has linked references to certain key words, images, or sounds. In the case of Maeve, she has complete control of her "hard disk" and can see that there is "something there just out her reach", this showing that she can scan her disk and see the data, but she cannot read the data because she doesn't know what kind of data it is. This is also true with any computer. You can see that there is something there, but until you know the structure of it, it doesn't make sense because you need a cross reference to put it into context. I.e she knows that she was built for a specific purpose, but the reference to where that information has been stored is not available to her. Not that it doesn't exist, but she needs a cross reference to it.

Source.