I have been experimenting with my new 1.5TB External Iomega Desktop Drive to see if there was an upper limit to the WBFS Manager as it didn't like the drive when I formatted the primary partition to 1.5TB FAT32.
Using the beautiful (free) Partition Assistant (http://www.extend-partition.com/download.html) I created a Primary 500GB FAT32 Partition and formatted the rest as NTFS.
This worked fine and WBFS Manager recognised the partition , formatted it and I was able to convert my Wii backups onto the partition and use them through UBS Loader GX.
Next , I deleted all the partitions and reformatted the primary to 1000GB with an unformatted secondary partition.
WBFS manager also recognised this and I was able to use it with my Wii.
However, any partition above this limit, craps out when using WBFS manager. So, I'd propose that an upper limit of 1TB is all you can use with WBFS Manager.
If I am wrong, please comment below as I'd like to be able to utilise the additional 500GB.
Also, I feel that it's worth mentioning that after re-sizing or re-partitioning a drive, I would always re-load WBFS Manager and always refresh the drive list.
I hope this saves someone a bit of frustration with drives > 1TB.