I have the problem that i have 2 HD and the first is quite full. I would like to use the second HD in Plesk.
Have you some instructions about using the second HD in plesk ?
Yes. At the moment. In my experience the second drive , with Plesk, is useful only to store backups. I have not any backup , then, it is useful for nothing. My problem is that with 100 domains , i have in /home/httpd/vhosts/ 25 GB of files php. I do not store any backup at the moment in the second drive. My problem is that if possible i would like to move some domains (the php files) from /home/httpd/vhosts/ in the first hard disk to a location in the second hard disk , but is it possible to change the dir that refer plesk from /home/httpd/vhosts/ to a location in the second server ?
What is the system to use the second HD in Plesk ? If it is not possible for me to use the second HD in Plesk the second HD is not useful for nothing. Have you some instructions about using the second HD in plesk ?

there isnt a way to do that without just moving domains and symlinking them. but i dont suggest that.

I am not completely sure how to do it. But you should be able to mount the second hard drive as /home2. Then from there add the sites to your control panel. But you would need to play with your panel a little to make sure it will create the domains there with out loseing the old domains.

If all else fails... copy the existing domains over to the second hard drive. Then create a symlink to their new location, where they used to be. That way your panel can create the new domains in the current folder. Make sure the webuser has permissions to the proper directories and when copying them to the second hard drive take care to copy them with the proper flag to preserve their permissions.

it is possible, but you may encounter some complexities.

If you are sure you are going to continue to grow. Get your second drive replaces with a bigger one. Then copy the sites over, and have the secondary replace the primary. Thus you end up with a bigger primary drive.

Alternative solution. done and worked

mounted 2nd drive to webusers directory of 1 domain and i use it to upload files by ftp.

Offer free backup hosting with hosting plans and store domain backups, and much more. Was the only way i could use 2nd drive with a domain properly.

Could also use for subdomains, and offer subdomain hosting, for many uses.

Just an idea, try at your own risk, but it worked for me.