cocksy_boy

After posting, I found an issue I had missed: the disk was marked as Read Only in disk management. 
After running DISKPART from cmd I managed to remove the read-only tag using the command attributes disk clear readonly  and it appears to be OK now.

After posting, I found an issue I had missed: the disk was marked as Read Only in disk management. 
After running DISKPART from cmd I managed to remove the read-only tag using the command attributes disk clear readonly  and it appears to be OK now.

Umfriend is correct. The service should be stopped to prevent any chance of balancing occurring during the migration when using that method.
And that method is fine so long as your existing arrangement is compatible with DrivePool's pooling structure.
E.g. if you have:
drive D:\FolderA\FileB moved to D:\PoolPart.someguid\FolderA\FileB
drive E:\FolderA\FileB moved to E:\PoolPart.someguid\FolderA\FileB
drive F:\FolderA\FileC moved to F:\PoolPart.someguid\FolderA\FileC
then your drivepool drive (in this example P: drive) will show:
P:\FolderA\FileB
P:\FolderA\FileC
as DrivePool will presume that FileB is the same file duplicated on two drives.
As Umfriend has warned, when it next performs consistency checking DrivePool will create/remove copies as necessary to match your chosen settings (e.g. "I want all files in FolderA to exist on three drives"), and will warn if it finds a "duplicated" file that does not match its duplicate(s) on the other drives.
As to Snapraid, I'd follow Umfriend's advice there too.

Moving data to the Pool while retaining the data on the same drive is called seeding and it is advised to stop the service first (https://wiki.covecube.com/StableBit_DrivePool_Q4142489). I think this is because otherwise DP might start balancing while you are in the process of moving drive-by-drive.
I am not sure but I would think you would first set settings, then do the seeding.
(I am pretty sure that) DP does not "index" the files. Whenever you query a folder DP will on the spot read the drives and indeed show the "sum". Duplicate filenames will be an issue I think. I think that DP measures the Pool it will either delete one copy (I think if the name, size and timestamp are the same or otherwise inform of some sort of file conflict. This is something you could actually test before you do the real move (stop service, create a spreadhseet "Test.xlsx", save directly to a Poolpart.*/some folder on one of the drives, edit the file, save directly to Poolpart.*/some folder on another drive, start service and see what it does?).
DP does not go mad with same folder names, some empty, some containing data. In fact, as a result of balancing, it can cause this to occur itself.
I have no clue about snapraid. I would speculate that you first create and populate the Pool, let DP measure and rebalance and then implement snapraid. Not sure though. You may have to read up on this a bit and there is plenty to find, e.g. https://community.covecube.com/index.php?/topic/1579-best-practice-for-drivepool-and-snapraid/.

You can reset the status, and it will flag it to get scanned. 
https://stablebit.com/Support/Scanner/2.X/Manual?Section=Disk Scanning Panel#File System Health
 

Okay, and as long as it's just the 2 disks, then you should be fine with doing this.
 
Once you have more than two disks, this gets a lot more complicated.

Actually, that is what I do: 2 x 2TB Pool, full duplication and I backup one of the two. Should I need to have more memory than I'll add another 2x2TB in a seperate Pool and backup _one_ drive of each Pool.