512KB? At the risk of going all Four Yorkshiremen, that sounds luxurious.
Floppy disks held 170KB if you were lucky to have a drive. The PET line, like many 8-bit computers, used a cassette tape drive (yes, those things that preceded CDs for holding and playing music). Capacity depended on the length of the tape. And it took ages to load.
The PET was fancy because it had a built-in cassette drive. That's what you can see to the left of the keyboard in the picture.