A thread from 2010: https://news.ycombinator.com/item?id=1337157

It was amazing hardware at the time.

What's incredible is you can now wear double the power of the Cray 2 on your wrist: http://apple.stackexchange.com/questions/194367/does-the-app...

By the way, Ed Thelen (whose website has the manual) also runs the IBM 1401 website (http://ibm-1401.info/) with tons of information about that old mainframe and the restored 1401s at the Computer History Museum.

    Each DD-19 has a capacity of 2.424 x 109 bits so that a
    maximum mass storage configuration could hold 9.7 x 109
    8-bit characters. 

Is that supposed to be 10^9?

Yes. There's a chart in the linked manual that states it with the superscripted 9.