Sata hardrive vs Noral hard drive

Has nothing to do with SATA. Instead, it has to do with binary vs decimal gigs. 320 decimal gigs is actually 298 binary gigs, which is what windows will see. 286 sounds a bit low, but if you got the computer from someplace like HP or Dell, there might be a recovery partition that's taking up the extra 10 gigs or so.

Don't forget some of the space is used when formatting the drive. HDDs are always rated by their unformatted capacity.

That space is insignificant though.

its about 12GiB of space missing, probably like cjl said, for the recovery

320,000,000,000 / 1024^3 ~ 298