Typically bytes are coded 8N1 (or 1 start bit, one stop bit, and 8 data bits) so 10 baud gets you 1 byte. Hence the nominal divide by 10 (this is 8b/10b encoding)

But in a pedantic sort of way, the computation should add extra bits for ECC since the drive has ECC bits already embedded in it, so a 1TB drive probably has more than 10 Tbits it is probably closer to 15 - 20 terabits.