nofail option in /et/fstab

You can specify even external USB disk drives or samba shares in /etc/fstab. Nevertheless you have to take care to specify the proper options because if the partition is not available duing boot the system will boot to rescue mode and you need to remove the filesystem from /etc/fstab.
The option you need to specify is "nofail". This will allow to boot the system if filesystem is not available during boot:

/dev/mapper/usbdrive      /media/usbdrive     ext3     auto,nofail,defaults     1 1