Removing a segment from a device has no effect on the data on the device. Extents that are allocated to objects on the segment remain allocated to those objects. The segment only affects which device(s) future allocations to the objects placed o the segment go. To actually free space on the device, you have to delete or truncate data from those tables, or cause the data to move elsewhere (once the segment is dropped from the device, rebuilding the clustered index on an APL table or running REORG REBUILD on a DOL table will cause all the data except for text/image/log to be moved to the devices that are currently part of the segment.
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