Complex sentence: Her most active duty during her career, was as part of the Flying Squadron of 1874-77, with the rest of her career spent as a powder hulk in Devonport, when in 1929 she was finally broken up.
Split into simpler sentences: Her most active duty during her career, was as part of the Flying Squadron of 1874-77. She spent the rest of her career as a powder hulk in Devonport until 1929 when she was finally broken up.