EJECT

mov bx, DriveNum  ; drive number, 0=default, 1=A, 2=B, ...
mov ch, 8         ; device category must be 8
mov cl, 49h       ; EJECT minor number
mov ax, 440Dh     ; generic IOCTL fuction number
int 21h
jnc success
mov [ErrorValue], ax  

This is a simple call through with no additional parameters. If the call is successful (CY flag is clear) then on return the specified drive will be ejected. If more than one drive letter is pointed to a single physical drive number, then the drive will remain locked as long as any of the relevant drive letters are locked.

• On error, sets the Carry flag and the AX register to one of these error values:

DriveNum (BX)

Specifies the drive for the operation. Can be 0 for default drive, 1 for A, 2 for B and so on.