First off, can you answer this: is this external unit supposed to be one 250 GB unit, or is it already configured to use whatever is inside as a RAID1 array? Some external units offer to - or are already pre-configured to - operate as a RAID1 array. If yours is not this way, proceed. (I really doubt this is your situation - I believe the unit you have contains only one Caviar Blue HDD, and it's VERY unlikely to have been configured as a RAID1 array with only one HDD unit.)
I assume for now that you do NOT have any valuable data on this new drive, and so you can wipe it out and start over. Yes? Do NOT do what I'm going to recommend IF you want to save anything already on that drive.
I suggest you re-do the Partitioning, etc. To start with, use Disk Management to Delete any Partition on the drive unit now. If there is more than one, Delete all of them until ALL of the unit is is Unallocated Space.
Now, Right-click on that Unallocated Space and choose to Partition and format the drive. BUT this time make sure you set the size you actually want - don't just click OK on all the pre-set choices. I presume you want the size to be the max you can get from this, probably about 230 to 235 GB. Then complete the task, similar to what you did before. When done,back out of disk Management and reboot.