> Macs measure file sizes in powers of 10 and call them KB, MB, GB.

That doesn't conform to SI. It should be written as kB mB gB. Ambiguity will only arise when speaking.

> Advertised hard drives come in powers of 10.

Mass storage (kB) has its own context at this point, distinct from networking (kb/s) and general computing (KB).

> When you've got a large amount of data or are allocating an amount of space, ...

You aren't speaking but are rather working in writing. kb, kB, Kb, and KB refer to four different unit bit counts and there is absolutely zero ambiguity. The only question that might arise (depending on who you ask) is how to properly verbalize them.