DOSBox, in an effort to emulate a class MS-DOS environment needs some location to emulate a virtual Hard Drive. Commonly MS-DOS users would be greeted with a
prompt once a computer is done booting. The hard drives currently in your system are most likely many times larger than the kinds of hard drives that existed back when MS-DOS was commercially available. Also, they contain software that is not suited for DOSBox. In order to create a realistic (and safe) environment to run your DOS software you should define a path somewhere on your hard disk that will be treated as a virtual hard disk. Note that upon starting DOSBox the prompt instead reads
which is a virtual drive in memory (RAMDrive) where the OS Tools are stored. Users cannot write data to the Z:\ and it exists only for DOSBox purposes.
Navigation between different drives is done by typing C: where C is the letter of the drive you wish to go to. By default DOSBox, like Windows, will detect Floppy Drives connected via floppy cables as the A:\ and B:\ respectively.
Mounting a C:\
It bears repeating that unsafe to mount one's entire drive into DOSBox, particularly if another OS is already installed on it. For that reason, it's recommended only to mount a subdirectory (subfolder) of the drive where DOS files will be stored and have DOSBox fool it's games/applications into thinking that is the C:\. Ideal locations for a virtual hard disk are
mount c C:\DOSROOT
mount c ~/DOSROOT
Note that you can have more than one Drive defined if you want to recreate complex configurations, but it is generally easier to have a single drive that will be mounted a your C: drive.