Getting the uid from user name for a view argument
Use the Data name argument to filter the results by. Each content field has it's own argument that you can choose to filter the results by.

(edit: reply to comment) Let's say that your drupal username is user3 with a UID of 3. The user ID of the external data will be user3? So when the user goes to mysite.com/user/3 the argument will be 3 which doesn't help with the external data because that needs to be mapped to user3? If the user ID of the external data is 3 then you can use data: field_userID or whatever it is and get arg(1) using provide default value from PHP code, page manager or Views Arguments from Context. Now, if you to map the Drupal username to the external data username/user ID which is user3 then use provide default value from PHP code, get the global $user variable and use $user->name for the argument. This maps the user's name to the database column in the data table and returns (filters) all results/ database rows with that username in that column.

Don't validate it until you get it working.

global $user;
return $user-&gt;name;


or

return arg(1);