Mounted FAT devices in UNIX System does not support permissions, hence SSH gives this warning:
$ ssh-add id_rsa @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ @ WARNING: UNPROTECTED PRIVATE KEY FILE! @ @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ Permissions 0777 for 'id_rsa' are too open. It is required that your private key files are NOT accessible by others. This private key will be ignored.
First you should take a second to think about the reason for this warning, once you did that you can bypass it like this:
$ cat id_rsa | ssh-add - Identity added: (stdin) ((stdin))
Have fun kids! 😉