A group at Microsoft is researching the potential of being able to store data in slabs of quartz glass, at densities comparable to hard drives or even higher https://blocksandfiles.com/2023/11/22/microsoft-silica-glass-based-storage/.

The data is encoded optically in the glass, using femtosecond laser pulses. There can be hundreds of layers of data within a single block of glass. However, reading back the data involves processing scanned images of the glass layers through a “machine learning algorithm”. This can take some time.

That means verification of a backup you just did is going to be just as slow.