I am using Apex Data Loader 18 to import records into a custom object I created.  My SDL field mapping file has a line that looks like this:

Attorney_Firm_Key=Account\:External_Key__c

My CSV import is structured correctly, the External_Key__c field exists in the Account object, and it is appropriately marked as an external identifier.

I know this general import method can work, as I have successfully imported our company contacts from an external system and linked them to their related accounts with the same "Account\:External_Key__c" value.

My problem is that running the import for my custom object fails with the following message:

"Invalid property name 'Account:External_Key__c'"

When I look at my Enterprise WSDL, the Account object has External_Key__c listed.

Again, I am confused that this would work for importing Contacts but not for importing custom objects.

I was able to temporarily work around this issue by using Import Custom Objects under Data Management in our SFDC org.