Is a zettabyte 1021 bytes or 270 bytes?
While is true that the theory goes as wikipedia says (1ZB=10^21 and 1ZiB=2^70), the truth is that "it depends".
Storage devices basic unit is not the byte, but sectors, which until recently (for compatibility) were always 512 bytes.
That means that a 64MB USB disk (as an example) will probably have the number of sectors to be greater than 1MB (SI), but will never be exactly that because of the basic unit being 512 bytes. This device will probably have 64*1000*512*2 bytes = 64KB * 1KiB = 65536000 bytes, which is not 64MB (64000000 bytes), nor 64MiB (67108864 bytes).
The same for normal hard disks, but their basic unit is usually greater than a single sector, although always a multiple of it.
A zettabyte are 1021 bytes = 1.000.000.000.000.000.000.000 Byte
270 bytes are called Zebibyte.
To quote from Wikipedia: