Product and version
FileMaker Server 14.0.4
OS and version
OS X Mavericks 10.9.5
Mac mini server late 2012
After an importation of records, with "Preserve external storage" checked, you will have the same path for the remote storage on destination table than source table.
But if you then delete the record on destination table, the external content will be deleted on the server , regardless if it is still referred by a record on source table.
How to replicate
Create a table A with external container. Let the path by default.
On the same database, create a table B with external container. Let the path by default.
Create a record on table A.
Import this record from table A on Table B, with "Preserve external storage" checked.
Delete record on table B
Go to table A and check for external content. The 404 error is shown. The file has been deleted.
Dont' check "Preserve external storage" on import dialog except on very specific situation. Since you cannot import form and to the same table, this issue will surely occur when the user will decide to delete a record.