As far as I know, the synology DSM creates more than one partition on the each disk. I don't remember exactly how many, but I remember having one or two 2GB partitions volume partitions on my disks when I connected them to my windows machine.
If you had a single disk setup, I would recommand you connect the "new" disk first, then use DSM to initialize the disk and create a volume on it. You can move the data from volume 1 to 2. Don't forget to point the applications also to volume 2, like home folders for instance. You can then disconnect the old disk and you diskstation will boot normally.
Posted via Xpenology.us