Depends on the implementation. Most I/O calls return errors when reads or writes fail, but NFS, for example, would traditionally block on network errors by def

Depends on the implementation. Most I/O calls return errors when reads or writes fail, but NFS, for example, would traditionally block on network errors by default — you probably don't want your entire lab full of diskless workstations to kernel panic every time there's a transient network glitch. You also have the issue of multiple levels of caching and when and how to report delayed errors to programs that don't explicitly use mechanisms like fsync.