Hi colindarling,
Welcome to Affinity Forums
We do not recommend editing files directly from external drives/cards/NAS (network drives/storage) and cloud synchronised folders as this may lead to lost of work/data due to loss of connection or files changes (in synchronised folders). Affinity apps don't load the whole file to memory for performance reasons and require a permanent connection/access to the file to ensure incremental saves/changes work properly. If the connection to the original file is lost we lose access to the required non-loaded data (to memory) to save the file to another location. Please use the internal system drive when working with affinity files. We are looking into ways to improve this.