Question about WD Red Plus

It is generally inadvisable (not a good idea) to combine different drive types and sizes in a single traditional RAID array. However, whether or not it is a good idea for you depends on your system setup and storage requirements. Some systems (e.g. Drobo) are designed to handle this elegantly and without issue. Alternatively, if you are planning to set up the additional drive as a separate, single disk volume on your NAS, it is not an issue since the new drive will not be part of a RAID array with the old one- though this way (to state the obvious) you will not achieve any redundancy or speed benefits.