EDIT: Moved thread to appropriate sub forum.
Here's a list of ISFS (IOS) error returns you could use to see what the exact issue is on any ISFS call.
https://www.wiibrew.org/wiki//dev/fs Scroll down to /dev/fs error codes
Also a plain jane ISFS_Read call returns the amount of bytes read (dumped to memory). It should always match the 3rd arg/parameter of when you called ISFS_Read.
Here's a list of ISFS (IOS) error returns you could use to see what the exact issue is on any ISFS call.
https://www.wiibrew.org/wiki//dev/fs Scroll down to /dev/fs error codes
Also a plain jane ISFS_Read call returns the amount of bytes read (dumped to memory). It should always match the 3rd arg/parameter of when you called ISFS_Read.