As reported by the mount command, there is no entry in your fstab for the USB disk partition. That is okay if you supply all of the necessary parameters to the mount command. You didn't give an example of your mount command and the resulting error message so I am just guessing that you do something like
I believe that when you restart Linux the startup process automatically mounts all of the disks that it can find. I suspect that this is why "...it works..." after you restart the operating system.
You should be able to mount the disk partition if you provide all, or most, of the parameters that would be supplied by an entry in the fstab file.
mount -t ext3 /dev/sda1 /mnt/sda1
Keep in mind that there are factors that you didn't mention. A cut-and-paste of the mount command with the resulting error message could be helpful. Whether you have automount running is a factor. So if my guess about the nature of the problem is incorrect I have to plead that you did not provide sufficient details to make a correct assessment of the problem.
In other words, it couldn't be my fault that I was wrong.
That is just not possible.