Permissions 0777 for 'id_key' are too open

What type of filesystem do you have on your USB stick? If it is one of the FAT filesystems, FAT8, FAT16, VFAT, FAT32, then you're probably out of luck, as they do not have fields to store Unix-style permission bits. (The UMSDOS filesystem driver for Linux does hack this up, but I doubt OS X has anything that hacky available. :)

If it is NTFS or NTFS2, then perhaps there can be some mapping made, but I wouldn't count on it.

If it is HFS+ or UFS, then definitely more details are needed, because it ought to work as you've described, but symlinks could get in the way.

This is a permissions issue on a Mac. The file system is HFS+, so the first comment is incorrect by far. Permissions are the basic foundation of the *nix OS, so it would behoove you to understand.

$ chmod 600 /Volumes/USB/id_rsa

I think chmod 600 /Volumes/USB/id_rsa is sufficient.