Excel won't allow you to directly reference an external data source for a data validation list due to security concerns. However, there are a couple of workarounds you can use to achieve a similar outcome:

Method 1: Using a Named Range and Refreshing Data

1. Prepare your External Data:
- Ensure your external data source is accessible (another workbook, text file, etc.).
- In a separate sheet (within the same workbook where you want the data validation), import or copy your external data.

2. Create a Named Range:
- Select the range of cells containing your external data list.
- Go to the "Formulas" tab.
- Click "Define Name" in the "Defined Names" group.
- Enter a clear and descriptive name for your range (e.g., "ExternalList").
- Click "OK" to create the named range.

3. Data Validation with Indirect Reference:
- Select the cell(s) where you want the dropdown list.
- Go to the "Data" tab and click "Data Validation" in the "Data Tools" group.
- Under "Allow," choose "List" from the dropdown menu.
- In the "Source" box, enter the formula `=INDIRECT("'" & ExternalList & "'")`. This formula uses the `INDIRECT` function to reference the named range by name ("ExternalList") and ensures it treats the range name as text (using single quotes).

4. Refresh External Data (Optional):
- If your external data source is prone to updates, you'll need to refresh the data and consequently the named range. There are two ways to do this:
- Manually: Right-click anywhere in your imported data and choose "Refresh Data."
- Using VBA code (for automatic refresh upon opening the workbook).

Method 2: Using a Hidden List and Refreshing Data

1. Create a Hidden List:
- In a hidden sheet (format the sheet properties to be hidden) within your workbook, copy your external data list.

2. Data Validation with Named Range:
- Select the range of cells containing your hidden list.
- Follow steps 2 and 3 from method 1 to create a named range for this hidden list (e.g., "HiddenList").

3. Refresh External Data (Optional):
- Similar to method 1, you can use the same techniques to refresh your external data and update the hidden list for the named range.

Both methods achieve the functionality of using an external data source for your data validation list. Remember to update the named range if your external data changes.

Additional Notes:

- Consider data validation with a limited list if the external data source is very large for performance reasons.
- For highly dynamic data sources, explore VBA code for more robust automated solutions.