SUMMARY:
While configuring and verifying an FTP device, an error message of “The FTP location is not overwritable” is returned because the user does not have read and write permissions to the directory specified.
SYMPTOM:
In the FTP Device Editor, when the option of “Overwrite files with the same name” is chosen, the following error is encountered:
The FTP location is not overwritable.

The error message received when the user does not have the overwrite privileges and the option of "Overwrite files with the same name" option is selected.
CAUSE:
When the "Verify" button is pressed, the Strategy Intelligence Server creates a random file in specified directory of the FTP Device Editor. If the user the Strategy Intelligence Server uses does not have delete privileges, the file can not be overwritten causing the above error.

This demonstrates how a random test file is created in the directory and the user does not have the necessary privileges to delete it.
The file will have to be accessed manually in the directory created in order to delete it. The below image shows the contents of the file:

This is the test file that gets created if the user does not have delete/overwrite privileges for the directory.
ACTION:
The user specified in the FTP Device Editor will need to have the delete privilege for the files to be overwritten.