My project consist of creating a mini video game console with a couple of emulators from different old retro game consoles. The machine has a Raspberry Pi 3 model B and a 2.5" external HDD (USB 3.0) connected to the Pi via regular USB. The software used is called "Retropie".
I powered the whole thing using a AC/DC 5V 2A power supply and, while everything runs OK, I had some issues while testing Play Station 1 games (which require more hard drive usage). Googling around I read that could be an amperage problem so I bought a 5V 3A power supply (which I attached a female USB connector since it didn't have any), and I had more issues than before: the Pi tells me every time there is an undervoltage issue by displaying a little thunderbolt icon on the software I am using (retropie). With the last power supply not only did the icon display every time but it also took a lot more time to boot up.
How can I make sure that there is enough power for the HDD all the time?