There have been a few concerns regarding the moving of data to a different drive if you rebuild an index in a partition scheme.

I want to provide a scenario to support the "may" in this option.

When partitioning a table, you create a filegroup for each partition. Once created files may be created for each filegroup. It may be necessary for each file to be assigned to a different drive. Let's say the table is 300GB and spread across 2 partitions and filegroups. Each of these filegroups has 10 files spread across 10 drives - that means each file is roughly 15GB, if we have partitioned the data down the middle (it's easier for this scenario).

Each partition data will remain in that partition when a rebuild occurs, however, due to the rebuild we have a scenario where the data could now potentially move from one file to another in the filegroup and thus be on a different drive.