Not with 100% reliability.
What you'd have to do is break and rebuild the array using your new disks. Then go through Online capacity expansion (OCE) to add capacity to the existing virtual drive. The added capacity can be presented to the operating system as additional space for the operating system to partition it as an additional drive, or it may be added to an operating system drive, depending upon the capability of the operating system. So it's really not making the existing drive larger.
For good reason, the RAID Software User's Guide says Warning: Before you change a virtual disk’s capacity, back up the data on the virtual disk, over and over.
You should have a tested backup anyway, so why not just create you new array with 3TB HDDs and restore from your backup to them. It'll be faster and safer.
I fully intended to break my raid with 2TB drives. The point is that I want to replace the 2TB drives with 3TB drives and use them to expand my existing raid that is using 3TB drives. Creating another array with new 3TB drives doesn't really accomplish anything to help this.
What creating a new array with your 3TB HDDs does is allow you to create a single "drive" to the operating system using the full capacity of the array you've created. Not one drive of the original size and another one that is the expanded capacity size.
What it also does is eliminate the possibility of array failure when you break it to remove a 2TB HDD and rebuild using a 3TB HDD and possibly encountering a bad sector on one of your remaining 2TB HDDs causing a two drive array failure.
It also seems like it could lower the time it takes to accomplish this to a great degree because you're not breaking and rebuilding over and over. Each rebuild can take a bit of time.
My statement still holds true about having a tested backup and creating a new array with 3TB HDDs and restore from your backup to them would be faster and safer. The method you use is your choice.
I'm just trying to provide safe suggestions. Most of which is having a tested backup before starting anything.
Sorry, I read your original message incorrectly. You can't "combine" two arrays into one array.
I still go with backing up your data (from both arrays) and creating a completely new array and restoring.
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This is 100% possible and will work. I did the same on my RT3WB080, though slightly different. The question is... do you need to save the data on the array with 2TB drives? If so, it's a little more difficult. If not, then it's extremely easy.
Not saving the data -- simply delete the array of 2TB drives, and then remove them from the server. Replace them with 3TB drives and then add them to the existing array of 3TB drives using the OCE feature. NOTE: make sure you have a backup of the data ESPECIALLY if you don't have a BBU.
If you need to keep the data from your old array of 2TB drives then you need to find a place to put the data first.
I encountered no issues expanding my array from 5 to 8 drives, though I had to do one drive at a time, and each drive took about 2 days to rebalance the data. After the week was over I now have a 21TB array of 8x 3TB drives (RAID5).
MAJOR NOTE: An issue you will most likely encounter is the partition size limitations of an NTFS volume. If you originally created the partition on your array of 3TB drives with windows default settings, you likely have an allocation unit size of 4K. The problem with this - having a 4K allocation unit maxes the volume out at 16TB. Your new array would be of 8x 4TB drives in RAID5 giving a volume size of 21TB, requiring a minimum allocation unit size of 8K or larger (8K maxes out at 32TB). Windows supports allocation units up to 64K for NTFS volumes giving a maximum single partition limit of 256TB (oh, the horror). Of course, the issue is, you can't change the allocation unit size once you've made the partition. SOLUTION: What you'd have to do is add the additional 4 drives to the array, then create a new partition using 8K allocation unit. The size of this would still be larger than the old 12TB partition (4 drives minus 1 times 3TB); the new partition would be 16TB, since the amount of parity needed still remains at 1 drive, giving you a 4 drive net gain. You then move all the data to the new 16TB partition, delete the 12TB partition, then expand the 16TB partition to 21TB to fill the whole array.
Hope that explains and helps to answer your question. As others said - have a backup, just in case. I didn't have a backup when I did this and ran into no issues, but I probably lost a week of sleep worrying I would during the constant rebuilds.