Ok, so there is the thing:

1. If you were not using Unity Catalog first, that means you have used totaly diffrent approach e.g. mouns.
If yes, you were accessing storage via dbfs path.

Once you switched to Unity Catalog you need to take cate of few things:
- the one you did - create Databricks access connector, assign blob contributor role to access connector (IMPORTANT: allow workspace clusters to access storage), create storage credentials, create external location

- as you mentioned you need to grant READ FILES permission over External Location for Service Principal that will be executing READ operation from you Storage Account 

- you also need to make sure clusters that are being used to execute the job support Unity Catalog and you don't use dbfs path but insted use same path (pattern) as you placed in External Location  

abfss://<container-name>@<storage-account-name>.dfs.core.windows.net/<path_to_file>

Remember: that if you run Workflow Job the Run As principal is the one that will try to "access" External location if not specified that will be Job onwer.