What is a Sector?
A sector is the single smallest unit of data that can be address by a computer, and as “standard” is 512 bytes long.
This does not mean that a computer can not read a single byte – the fact that you can type disproves this – but this is the smallest unit of data that be read in at a time.
A standard hard drive is a SCSI, S-ATA, or IDE, and it be on a Apple Mac or PC it makes no difference, as long as its nothing to do with IBM AS400!
While a single sector can be accessed by a computer and specific forensic tools, file systems write to “clusters”, which are a group of sectors.