I have noticed on new repository creation for attachments, the default check box would not be able to make active unless the old repository Default value is unchecked. This at time is clumsy to go back, find the one which is set as default repository Uncheck the value and make the new repository the new default Check box
Steps to point the issue:
1. Login into ServiceDesk as Administrator
2. Select Attachments Library --> Repositories --> ServiceDesk
3. Click Create New button
4. By design the Default value is set to 'No'
5. Make this as 'Yes' and fill the necessary fields - Click save
6. You will see the warning message "AHD10319:A repository with the same type is already set as default."
This should be corrected automatically by setting the old Repository to value 'No' and the current one to 'Yes'
Hope this helps.