I´ve already replaced the internal DVD drive in my MBP 17" 2011 with a harddisk. But why not adding more?
There is only 2 internal sata ports in a Macbook Pro, this is maxed out.
But there is 3 USB ports (besides thunderbolt), which can be picked off from the solder joints on the logic board.
And there is this HUGE battery of 7hrs that could be replaced with a smaller one, lasting maybe only 30 Mins. I never needed more time when being "unplugged"
So this mod could make space for some additional 2,5" harddrives, giving a total of 4TB. The case work is easy.

Now, it sems that the Macbook does not only need the battery for keeping preferences like date and time , but also uses it´s extra power when doing cpu-intensive tasks. My battery can be down to 85% after doing heavy rendering for one night, even though the Mac was plugged to AC.
So I guess a smaller battery of the same voltage is still necessary.

Here are some questions:

-What are the exact voltages of (I guess there are several) of the battery?

-Does anyone know the pinout of the 10-pins battery connector?

-Does the system measure only the voltage of a battery when loading, or does it also measure the amperes/power?

I want to solder the USB-cables of the extra harddisks to the solder joints of the USB-connectors on the logic board. I guess figuring out the pinout is very easy there. But there is not much room for cabling because of the tight space.
Only very thin wires or a flexible flat cable can be used. Do USB specifications allow such cables over a distance of around 15-20mm?

Thank you for some hints, all the best, Herbert