I’m sure it’s a mix. Back when it launched there were a number of rumours about it being Blu-Ray based. They had similar capacity for the space used compared to

I’m sure it’s a mix. Back when it launched there were a number of rumours about it being Blu-Ray based. They had similar capacity for the space used compared to tapes, were considered very physically stable storage mediums, but had long access time as they would need to be physically moved, like tape, explaining the retrieval times.