Everything your Debian system will ever use is found on one file system or another. When the system is first initialized, the kernel uses the file /etc/fstab to mount any additional file systems needed. The root file system is initially mounted read only by the kernel so it can read /etc/fstab. One of the things that this system file tells init, is which file systems need to be checked, and in what order. Normally the root file system is checked first, followed by any other file systems that need checking. init uses fsck to check the partitions it finds listed in /etc/fstab. After the file system check is completed, the kernel re-mounts the root file system read/write and proceeds to mount the other devices it finds in /etc/fstab.
If you want a device permanently mounted on the system, simply make an entry in /etc/fstab specifying the mounting parameters and then execute the command:
mount -a
which causes all of the devices listed in /etc/fstab to be mounted.
Additional devices can be mounted and unmounted as needed using the mount and umount commands. To mount an ATAPI/IDE CD-ROM on /dev/hdd use the command:
mount -t iso9660 /dev/cdrom /cdrom
The device can be unmounted by either specifying the device (/dev/cdrom) or the mount point (/cdrom). So either of the following commands will unmount the CD-ROM drive:
umount /dev/cdrom
umount /cdrom