The old drive had 16 billion usable bytes. The new drive has 15.4 billion.
Base 2 has nothing to do with this problem.
> [...] Operating Systems define 1 GB as 1,073,741,824 BYTES.
Mac OS, iOS, Ubuntu, and Debian operating systems at the very least all use base 10 for representing disk and storage space.
Linux (to include Android) and the BSDs (to include MacOS, and iOS?) use GB (gigabytes) to mean decimal GB (gigabytes), a conversion measurement basically noone in the real world adopted because multiples of 1000 are meaningless.
So the vast majority of people and Windows to this day understand a kilobyte as 1024 bytes, a megabyte as 1024 kilobytes, and so on. Meanwhile, Linux and the BSDs and drive manufacturers/vendors understand a kilobyte as 1000 bytes, a megabyte as 1000 kilobytes, and so on because it's pedantically correct (and the conversion factors are commercially convenient).
This dissonance in understanding leads to endless "why is my drive smaller than what's printed on the box?" complaints.
As I undertood it, that tidbit comes from Sandisk and probably targets Windows user. I also believe Windows does (or did) use base 2 for file and disk sizes
G has always been 1000^3, M 1000^2, and K 1000, in the storage and communication industries. OS designers, and programmers more generally, started using 1024 instead for convenience, but that came later. The storage industry is doing it right, using the correct meaning of SI units, and programmers co-opted GB to mean 1024^3, it isn't the other way around.
There were times when the storage industry and programmers worked together to really stuff things up and cause further confusion by mixing & matching: the 1.44MB of HD 3.5" floppy disks was actually 1474560 bytes so 1.44*1024*1000.
Even then it was only the marketing portion of the storage industry playing that game, when not talking to the general public floppy disks were referred to by their maximum rated information carrying capacity, 2Mbyte, not their OS-formatted capacity at all.
It's frustrating that SanDisk used to give you extra bytes and they stopped -- and everyone including me HATES it when products get worse with no external indication that they changed.......but let's be honest: it's kind of SanDisk's primary MO to buy the cheapest NAND they can find and sell it on the consumer market.