not a clue how this is done with DA, and im failry sure its not possible to do without a fait bit of work... you need someone who has either done this kind of thing before with DA, or speak to DA support to check how it can be done...
Once you install the 2nd HD, you'll need to mount it as /home2 or something like that. Then copy the user files over to /home2/username. Once that is done, you can use usermod to move his home directory:
The rewrite is so that httpd finds the new home directory. You might also get a few issues with the ftp paths, which will have to be changed as well. (/etc/proftpd.passwd)
I have done this before and it has nothing to do with Direct admin and works fine for me.
For example is your user has a directory like this
/home/user1
then you will want to install you new hard drive and mount that and create a folder on it (example../user1)
Next you want to copy everything over from
/home/user1 to new hdd /user1 folder you just created
next create a symlink from /home/user1 to point to new hdd /user1
You need root to be able to do any of this. I havent had any problems at all with mine.