Error message using USB storage

I'm new in Linux and need help to use USB storage.

I followed Using storage devices without errors and see the mount point /mnt/sda1 in LuCI.

I trry to make a directory on the usb storage device with mkdir /mnt/sda1/rrd and receive the error message:

Command 'mkdir "rrd"'
failed with return code 1 and error message
mkdir: can't create directory 'rrd': Bad message.

What am I do wrong?
I need the strorage location /mnt/sda1/rrd to use for the RRDTooll during setup LUuCI statistics.

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?

where to look for logs?
I can cd /mnt/sda1
The problem persist after a reboot.

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

What can I do?

I don't know how but I can now use the external storage and make a directory.
apparently the error messages are not fatal.