You may have to provide a little more detail on this. The first thing that comes to my mind is that the partition becomes unmounted. This would happen when you reboot. A related idea is this: If your system mounts other drives by UUID, but you have listed your share the traditional way, i.e. /dev/sdx, udev may be changing the dev/sdx to something else. Hence, sometimes when you boot, you will be able to access the drive, other times you won't. Check your /etc/fstab file and see if they are listed differently.
Of course, this is complete speculation, but it is what popped into my mind.
Last edited by liberalchrist; 02-23-2012 at 08:44 PM.