
Upgrading your Mac's storage internally is pretty much impossible these days, with Apple operating a very tight ship to keep internal repairs and upgrades off the table. The only way to upgrade your Mac's storage without adding a bulky external hard drive to your setup is to use an SD or Micro SD card. The former will plug into a MacBook Pro SD slot with ease, while the latter can be easily adapted with something like a Nifty drive.
Then you can apply much cheaper flash storage to your Mac, without adding bulk. Right now SanDisk’s 400GB microSD card is in the Black Friday sale, down from $69.99 to just $48.