LOCK/UNLOCK

mov bx, DriveNum  ; drive number 0=default, 1=A, 2=B ...
mov ch, 8         ; device category must be 8
mov cl, 48h       ; LOCK/UNLOCK minor number 
mov ax, 440D      ; generic IOCTL function number
mov dx, seg ParamBlock
mov ds, dx
mov dx, offset ParamBlock
int 21h
jnc success
mov [ErrorValue], ax

Locks or unlocks the drive.

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

01h	Function not supported
B0h	Volume not locked in drive
B2h	Volume not removable
B4h	Lock count exceeded

DriveNum (BX)

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

ParamBlock (DS:DX)

Address of the parameter block for the function. The block has two byte fields. The first is LOCK_UNLOCK_OP, which has one of the following values:

0	Lock volume in drive
1	UnLock volume in drive
02	Return lock/unlock status

The values 03h through 0FFh are reserved.

The second byte field is filled on return from the call as the lock status of the drive.