That should not be happening. What is happening is an empty partition from the USB disk is showing up on the internal disk. Udev is confused about the partitions. Here are a few things to try:
Start by unmounting both disks, then click on the double arrow icon. Click on each disk check file system icon (check mark by the partition). See if that fixes the issue.
Shutdown the server and disconnect the internal disk. See if the USB disk is recognized.
If it is, unload the internal disk files and reformat the disk.
Copy files back to the internal disk.
If not, unload the USB disk and reformat that disk.
Use UD to do all the partition removal and reformatting of the disks.