If you do a Factory Reset with an eSATA enclosure attached the MSS will format it. Otherwise it will NOT. When you install it you will have to choose between adding it to the pool (when it WILL get formatted), using it for backing up Shared Folders or leaving it unmanaged. I have not found a need for any special drivers for any eSATA enclosure or drive yet nor have I heard of one being needed. I do not have an "500GB DVR Expander eSATA external hard drive" however, so your mileage may vary!
When you plug in the drive just do not add it to the pool or add it as a backup device and you'll be fine.