In the episode "Collective", the Voyager crew encounters a mostly abandoned Borg Cube. Previously, the crew went to a lot of trouble to steal a Transwarp coil from a Borg sphere. Why didn't they take advantage of this opportunity to steal another one?
The Borg ship was shielded from transporters for almost the entire time that Voyager was in its vicinity.
TUVOK: Their shield generators are too deep inside the vessel. Our weapons can't reach them.
It's only after Voyager overloads the cube's power systems that the shields drop. The cube is evidently in such dire straits that it's within seconds of exploding.
LEADER: Their feedback pulse is overloading the induction grids.
SEVEN: This vessel will be destroyed. We must evacuate.
Which leaves us with two possible outcomes;
- Either Seven was right and the cube did eventually explode (off-screen) in which case there would have been no useful salvage or
- The cube didn't explode and a salvage mission was conducted but the damage from the power system overload was so severe that there were very few salvageable parts (such as the partial database she was able to extract).
Either way the transwarp coils are liable to have been damaged beyond repair.
The damaged cube kept the Delta-Flyer and multiple crew members hostage and held Janeway and Voyager off for hours mostly by bluffing. The crew never really had a chance to explore the cube.
See the Memory Alpha page on the episode for more information.