fstab on boot: mount when device is plugged in

You want the nofail mount option.

man mount

nofail Do not report errors for this device if it does not exist.

change the auto option to noauto. This will not mount the device on startup, while the user option will mount it the moment you try access it from your file manager, i.e. mount on demand.

UUID=myawesomeid /media/usb1 ext3 rw,async,user,noauto 0 2

More on fstab options here.

Alternatively

No need for a fstab entry when udev/dbus/hal can auto mount removable devices for you. See The Manual on setting one of these up.

One configuration I find helpful on minimalst WM's like Openbox, is to run an auto-mounting FM as a daemon to handle removable devices for you. One example is to include thunar --daemon in your startup script.

As the exter fs is ext3, it will contain permission bits. Suggest taking ownership of the device if you are not the owner already:

sudo chown -R $USER /media/usb1