Your data is always on the 3th partition of a disk. So disk 2 will be /dev/sdb3 If the disk is single mounted then SSH into the NAS and perform
Use df to see where /dev/sdb3 is mounted on. And what the device name of your new disk.
If this is i.e. /share/MD0_DATA then perform # cp -rf /share/MD0_DATA/* /dev/sde3/. Replace /sde3/ with the name of your device. This should copy all data from the share of disk 2 to the disk 5 and will create also a share with all dir's and sub dir's. It will still take a lot off time because it's about 1.75 TB.