Yes, unionfs is nice. I use it frequently to 'modify' iso's before burning them. Your existing /data/mail directory on hdc could be renamed to /data/.mail_orig. You could then create a new directory on hdd named /data2/.mail_new. Create a union of .mail_orig (readonly) and .mail_new (readwrite), and mount the union on hdc:/data/mail. You then do all your access through this new /data/mail unionfs directory, which looks just like your original directory, except with more space. If you try to modify a file that already exists on .mail_orig (readonly), it is auto-magically moved over to .mail_new for you. If you delete an existing file from .mail_orig, it is not really deleted (readonly), rather it is 'whited-out' over on .mail_new so it appears deleted when viewed through the /data/mail unionfs directory. Really slick. You never touch .mail_orig or .mail_new directly (you can, but probably don't want to for this scenerio). That's why I showed naming them with a leading dot to make them hidden directories.
Extending a Partition or Filesystem in Linux without LVM extending partition linux without lvm, linux extend non lvm partition, resize linux partition without lvm, extend non lvm root partition We may come in a situation that running out of space in a virtual machine Linux guest OS which doesnt have LVM structures. Resize Or Remove Logical Volumes With LVM 14th February, 2017 by Kelly Kirkham When working with logical volumes in Linux for file systems, one of the big advantages that it brings is the ease of resizing volumes along with the ability to do so on the fly.