Let's say you have a directory /mnt/raid which has files and directories inside it, but nothing is mounted to it.
Then you mount a block device such as /dev/sdh to /mnt/raid
Even though /mnt/raid has files and directories there, you can only see the mounted contents of /mnt/raid.
Just do a bind mount of the root filesystem to another location.
mount --bind / /bindmount
Now just go to /bindmount/mnt/raid and you will be able to see the original contents of those files/directories and this is the case with any other situation where something is mounted on a directory.