From a letter about the McGuire family history written in 1904 by Elizabeth McGuire Trosper:

My father... crossed the plains in '49 with six children, two of his sons were married and living in Iowa. They remained there until 1852 when they came to California with ox teams... [his father and mother also came out with them] They settled in Santa Rosa valley and lived a good many years. They had seven children, six boys and one girl.

This leads to a confusion on whether the family had come to California in 1849 or 1852, though in Elizabeth McGuire Trosper's diary entry from May 3, 1896, it indicates that the family arrived at Peter Lassen's Ranch on October 18, 1849.

In the biography for his son-in-law, Thomas Gilbert Woolsey Trosper (History of Sonoma County California, by Tom Gregory, pp. 486-488), it is written:

... James McGuire brought the family to California in 1849, crossing the plains with ox-teams and located at Knight's Landing, where he died, the family afterwards removing to Green Valley, Sonoma county.

Knight's Landing on the Sacramento River in Yolo Co., CA was settled in 1843 by William Knight, but the township was not given that name until 1853.

Elizabeth indicates in her May 3rd entry, that the family had lived at Knight's Landing only during the summer of 1850 (having spent the winter in Fremont in Santa Clara Co.) and that her father had died while the family was living in Vacaville, Solano Co. in January of 1851 (she lists January 12, 1851 elsewhere) The family moved at the end of the summer of 1850 from Knight's Landing to the Napa Valley. They stayed there for two weeks and then moved to Vacaville. The McGuire family continued to live there after James had died. In 1853, they moved to the Santa Rosa Valley. In 1856, they moved near Occidental, stayed there until 1864 and then moved on to Austin Creek in Cazadero.