> Even 4MB would take you hours to load from floppies with a 6502.

Depends on your clock. Also, you could use some dedicated hardware, like a DMA controller e.g. 8257, or 8237. From 8257's datasheet:

    Speed

    The 8257 uses four clock cycles to transfer byte of
    data. No cycles are lost in the master to master transfer
    maximizing bus efficiency. 2MHz clock input will
    allow the 8257 to transfer at rate of 500K bytes/second.

Hard drivers were available for the 6502. They were expensive ($10k for a 10MB drive as I recall prices came down a lot, but never affordable in the 1980s)

Processing terabytes with a single CPU was impractical, but you could in theory connect it.