moving /usr, /var, and /etc

[Antony <ntonyworkshop@gmail.com>]

I have a Debian system on really small Flash memory like an embed system. A hard drive is mounted onto the system. I want to add more processes to the system like mail server. But the problem is /usr, /var, /etc are running out of space on the Flash memory. I'm planning to do the following:

- make /usr, /var, /etc, /home directories on the mounted hard drive.
- copy /usr, /var, /etc, /home original directories to new created directories.
- edit /etc/fstab to mount these directories to new created locations.
- remove the old /usr, /var, /etc, /home directories.

Is that unsafe to do so?
Do files in these directories being accessed before mounting from /etc/fstab?