wo things that comes to mind:
- 1TB is NOT counted as 1*1^12 bytes (because 1kB counts 1024 bytes, and it adds up for larger volumes).
- Small files takes space too. Therefore may small files will fill more space than the sum of their size.
That was my first thought too. A 4TB drive should have approximately 4,000,000,000,000 bytes of storage, but Windows uses the kilo, mega, giga, and tera prefixes to refer to groups of 1024, resulting in such a drive appearing as having a little under 3.64TB to Windows. It's the same amount of usable space, just referred to in a different way.
I noticed just now that this was apparently for MacOS though, and I believe Macs have switched to using the same 1,000-based prefixes as hard drives when referring to file sizes and drive capacities.
And if these files were in fact copied from a 3TB drive, then any potential difference like that should apply to that drive as well, and there should still be at least 1TB of usable space available (or at least 931GB as reported by Windows), not just 270GB. So either that other drive was larger than 3TB, or perhaps some cloning method was used to copy the files over that left some space on the new drive unformatted, or something else along those lines.
So, questions worth asking... Do you still have the original "3TB" drive, and if so, how much total space is it reported as having, and how much of that space is reported to be used and free? And the same for the new drive?
And also, how did you transfer the files from the old drive to the new one? By just copying them over through the file explorer (I believe Finder on a Mac), or by using some other software to clone them from one drive to the other?