I have the SandDisk 400GB card in mine, and […] it's working great.
Cool! Now I just have to decide whether to buy one immediately, or wait for a decent 512GB one…
it contains 366.73 GB of usable space.
That's just due to the different ways of measuring.
Technically, the S.I. prefixes are always strict powers of 10: ‘G’ for giga = 10^9, ‘M’ for mega = 10^6, &c. But for years they've been abused by everyone for bytes, to mean powers of 1024. Or rather, by
almost everyone: storage manufacturers mostly
use the strict definitions, because it makes their disks look bigger!
There's already
a solution to this: new S.I. prefixes ‘Gi’ for gibi = 1024^3, ‘Mi’ for mebi = 1024^2, ‘ki’ for kibi = 1024. (The ‘bi’ is for ‘binary’.) These are catching on slowly, and once everyone's using them, there will be no confusion!
In this case, 400 GB is about 372.53 GiB, so you haven't lost as much as you might think to formatting and hidden partitions.