Stack of hard disk platters (what's inside a disk drive), I've always thought. Bit more modern than drum (where AIUI the storage surface is the outer circumference, like a phonograph roll); bit less abstract than "a barrel of data".

Oracle used to have logos for some products that looked like that (or, come to think of it, like a stack of button-cell batteries):

https://www.google.com/imgres?imgurl=https%3A%2F%2Fvtcholdin...

And I've always wondered if they based their HQ building on those logos, or if it was the other way around:

https://www.google.com/imgres?imgurl=https%3A%2F%2Flive.stat...

I've always thought of those as more like barrels of data (as opposed to oil).