I have a Drupal website on a VM (RHEL 7) and it works on an SSD storage both public and private directories: /storage/drupalpublic and /storage/drupalprivate.Now and in the future, I need to store a lot of gigabytes of data, but the SSD is definitely not a good way because of the price.Can I attach a regular hard drive and mount it like data1 over the private folder like that: /storage/drupalprivate/data1?Is it OK or will there be some interference between the SSD and regular hard drive?Do I have to place them in a some particular order in fstab?Will drupal understand all of that or it does not matter for it?
Mount a disk over another disk
filesystems;mount;drupal
disks can be mounted on any directories, there are however pitfall.all disk must be mounted before application (e.g. drupal)  is started.deepest directories must be mounted last (e.g mount /storage/drupalprivate/ before /storage/drupalprivate/data1 ).any existing file or dir under /storage/drupalprivate/data1 on your SSD disk, will be unavailable/hide once you mount /storage/drupalprivate/data1 from other disk.your /etc/fstab will look like (among other lines)/dev/sdb            /storage/drupalprivate/              ext4    defaults        1 2/dev/sdc            /storage/drupalprivate/data1         ext4    defaults        1 2assuming /dev/sdb is SSD disk and /dev/sdc is 'normal' disk.