Remember in the film, when Hiro & Baymax went into the portal to rescue Callaghan's daughter, Abigail? Well, in that scene, Baymax's armor got damaged when some debris hit him, damaging his thrusters.
"There is still a way I can get you both to safety." Baymax said, as he put his rocket fist into the bottom of the pod with Abigail inside it.
Hiro stared at Baymax. "No, no, no, no, no, wait." He said, shocked. "What about you?"
"You are my patient." Baymax said calmly.
Hiro tried to stop Baymax. "B-B-Baymax, No!" "Your health is my only concern." Baymax said.
"Stop!" Hiro said. "No, I-I-I'm-I'm gonna figure out-"
"Are you satisfied with my care?" Baymax asked.
Hiro was getting frantic now. "No! There's gotta be another way! I'm not gonna leave you here! I'll think of something!"
"There is no time. Are you satisfied with my care?" Baymax asked.
"Please, no!" said Hiro, now on the brink of tears. "I can't lose you too!"
"Hiro...." Baymax said, "I will always be with you."
Hiro tearfully hugged Baymax. He could feel himself crying, over losing first Tadashi, and now Baymax. Baymax hugged him back.
"I am satisfied with my care." Hiro said, heartbroken over losing Baymax. But he knew it was for good reason.
With that, the rocket fist blasted off, pushing the pod and sending Hiro & Abigail back to San Fransokyo.
Hiro could only watch sadly as Baymax drifted off into the darkness of the portal.
And sometime after Callaghan's arrest, Hiro rebuilt Baymax, after discovering the healthcare chip.
But little did he realize was that he would see the first Baymax again, sooner than he expected....

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The Story of Yamamax
FanficNightmare Crown midquel. This is the story of how Yama's henchbot, Yamamax, the evil double of Baymax was created. Spoilers for Big Hero 6 are in the first chapter. Set during most of the Nightmare Crown story, some of the prequel, and the ending of...