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The "U"-shaped white structure at the center of the picture is the Rigid Array Carrier, a pallet which will be mounted in the Orbiter's cargo bay. The silver boxes are the carriers for the instruments to be installed during the next servicing mission.

The orange panels at the corners under the RAC are air bearing tables, used to move the RAC for shipping.

NASA picture sts109-329-021 shows the RAC mounted in Columbia's cargo bay, loaded with Hubble's solar arrays.

Page 4 of STS-109 Hubble Space Telescope Servicing Mission-3B flight Readiness Review [direct link to 252K PDF] shows the location the RAC took in the cargo bay on the previous servicing mission.