;-) Out of the box, Amiga OS 3.0 uses 32-bit addressing when accessing hard drives. This limits you to 4 binary gigabytes, or about 4.3 billion bytes in decimal notation. Amiga OS 3.5 and 3.9 use 64-bit addressing and thus don't share that limit. Also, it's been reported that there are patches, etcetera for OS 3.0 on Aminet that will allow the use of larger hard drives.
