FX-6, serving as an FX-9 surgical assistant, constructed Grievous after the Kaleesh suffered a horrific shuttle accident that nearly resulted in his death and obliterated the majority of his initial physical form; the assistant is described in more detail in the FX-9 surgical assistant article.
In 19 BBY, FX-6 was on Coruscant at the Grand Republic Medical Facility alongside a DD-13 medical assistant droid, 2-1B surgical droid, and another FX-9 surgical assistant. During the rebuilding of Darth Vader after his defeat by Obi-Wan Kenobi on Mustafar, it carried out blood transfusions. Sometime after 12 BBY, FX-6, along with the DD-13 and 2-1B medical droids, were relocated to Vader's fortress on Mustafar; there, they kept enhancing Vader's armor according to his directions.