# OPL and disks

Memory disks in the `D:` drive may be used in the same way as the internal memory by specifying `D:` where you would usually specify `C:`.

There are two main reasons for using disks:

- To provide more room for storing data.
- To make backup copies of important information, in case you accidentally change or delete it (or even lose your machine).

The internal memory, memory disks use a FAT directory structure, the same as that used by disks on PCs. For more details, see Advanced Topics.

The ROM drive, `Z:`, can also be read from. However, `Z:` is read-only.