In order to migrate from synology to WD, I used 2 hdd one 2TB and one 3TB and I created a raid 1 with 2 TB volume size.
After the copy of my files, I switched the 2TB hdd with one with 3TB.
I am trying to expand the raid but I can’t.
There is a specific subforum for the EX2. Generally this subforum (My Cloud) discusses the single bay/single drive My Cloud unit which is a different device. Users in the EX2 subforum may be able better assist with your issue as they may be more familiar with hat specific WD device. One can also, using the forum search feature (magnifying glass icon upper right) search through that subforum to find similar discussions to your issue.
If you haven’t done so already see the My Cloud EX2 user manual page 87 for an explanation of what each RAID setting does: http://products.wdc.com/library/UM/ENG/4779-705119.pdf#page=87
There is WD Knowledgebase article on RAID that details some RAID issues
My Cloud RAID Online User Guide and Solutions
Because you are changing the size of the RAID disks you may have to either backup all of the existing data on the current drives, then delete the RAID and create a new one using the larger sized drive, then copy the data back to the newly created RAID. Or try a manual RAID rebuild. However, not sure a manual RAID rebuild would expand the size of the raid to fill the larger drive space.
Generally when using RAID 1 one uses the same sized drive for both hard drive due to the nature of what RAID 1 is doing.