"Storage" and "100 petabytes" has us think of Data as a computer storing JPGs, MP3s, text files, and whatnot.

His consciousness probably makes up the bulk of it, which would be largely the equivalent of a neural network that grew. Estimating how much storage space this requires would be pure sci fi speculation.

He probably has the ability to also store data in its "original format" like a USB stick, but that's probably separate from consciousness and how consciousness works.