Shared iTunes library: airport extreme and networking trouble

Hello, I have a question how to permanently share the iTunes library via an airport base? time capsule or airport extreme The problem is not the sharing option, but that my macbook pro, with mountain lion 10.8.2, does not recognize sharing after reboot Of course, the base is permanently connected, the local network permanently works and hard drive where the library resides, is always attached to the base's USB port. I did not store it into the time capsule's internal drive as I always take my hard disk with me when i go out of home and rattach it when i come back. well, from iTunes, I set the library so that iTunes recognizes /volumes/portable/iTunes as default iTunes directory from preferences, advanced. the trouble is that every time I reboot the mac, I must mount the disk! If I forget it, and launch iTunes after just having rebooted, the new podcasts, apps and so on, are stored again into /users/talksina/music/itunes... and so on. that is, on my macbook's hard drive. For mounting the disk, I mean these operations: command shift K on the finder, to open the local network resources then, selecting with arrows the desired airport base, then going right, and selecting the hard disk. then close with command+v. It means, the hard disk, that i named "portable", must be seen on the mac's desktop is there a way to solve the problem? thanks!