Is there anything useful in the logs?
Can you cd /mnt/sda1?
Does the problem persist after a reboot?
Does the problem persist if you use a different USB drive?
I started with a new on Windows formatted USB drive and now followed the document Quick Start for Adding a USB drive
In step 9 /etc/init.d/fstab boot I get the following error messages:
block: No "mount.ubi" utility available
block: mounting /dev/mtdblock7 (ubi) as /mnt/mtdblock7 failed (2) - No such file or directory
block: No "mount.ubi" utility available
block: mounting /dev/mtdblock8 (ubi) as /mnt/mtdblock8 failed (2) - No such file or directory