If the drive has not completely failed, you must wait for data migration to complete. If you physically remove a disk before data migration is complete, the data is not correctly replicated to the replacement drive.
If you are replacing a drive that has failed completely, the drive will be marked offline automatically. You do not need to wait for data migration to complete. You can replace the drive immediately.
If you are proactively removing a drive or the drive has not failed completely, you must wait for data migration to complete.