Type: Internal (3.0 and later)
Purpose: Used in the CONFIG.SYS file to set the maximum number of drives that can be accessed.
If you set the available disk drives to a letter value lower than that required by your existing system, DOS will automatically override the letter value to accommodate the drives installed in your system. You should only increase the number of drives if you are actually going to be using additional drives because each drive letter above E decreases the amount of RAM available for other purposes. You do not need to use LASTDRIVE to assign a drive letter to devices that are loaded in your CONFIG.SYS file. Option drive letter – Specifies the last disk drive (A to Z) that will be available. The default is E or the last drive specification in use on your system.
To increase the number of drives that can be accessed by DOS to 4, enter the following lastdrive=d