'Kilo' means 10 to the power of 3. (i.e. 1000). However computers use binary arithmetic. Those who work in the field quickly decided that it would make more sense to interpret 'kilo' as meaning 2 to the power 10 (=1024), rather than as 1000. Following on from that, 'Mega' came to be interpreted as 2 to the power of 20 (= 1,048,576), rather than as 1,000,000, for computer purposes. As the numbers used by computer designers got bigger, it then followed on that 'Giga' came to mean 2 to the power of 30 (= 1,073,741,824), rather than as 1 billion.
So a hard drive which has 1500 billion (real) bytes will have {1500 divided by 1.073741824} (computer) Gigabytes, which is about 1397 (computer) GB. That would partially account for the apparent deficit which you've reported. However I suspect that the '1500' in the description might actually refer to 1.5 x 2^10, rather than 1.5 x 10^3. In which case, a factor of 1.024 needs to be brought into the calculation, giving a disk size of roughly1364 (computer) GB. That would certainly be the case if the drive was actually described as '1.5 Terrabytes'.
It has always been the convention that hard drive manufacturers will report the true number of bytes available on their products, even though the computers that those drives are placed in will then report a lower figure (because the computer requires 1024 bytes to make up one Kilobyte, rather than 1000). As drive sizes get bigger, the differences become more marked.
Chris