; Update the first line below to match the desired drive letter (you can ignore all the other lines below).
Driveletter = I:  ; Set this to the drive letter you wish to eject.

hVolume := DllCall("CreateFile"
    , Str, "\\.\" . Driveletter
    , UInt, 0x80000000 | 0x40000000  ; GENERIC_READ | GENERIC_WRITE
    , UInt, 0x1 | 0x2  ; FILE_SHARE_READ | FILE_SHARE_WRITE
    , UInt, 0
    , UInt, 0x3  ; OPEN_EXISTING
    , UInt, 0, UInt, 0)
if hVolume <> -1
{
    DllCall("DeviceIoControl"
        , UInt, hVolume
        , UInt, 0x2D4808   ; IOCTL_STORAGE_EJECT_MEDIA
        , UInt, 0, UInt, 0, UInt, 0, UInt, 0
        , UIntP, dwBytesReturned  ; Unused.
        , UInt, 0)
    DllCall("CloseHandle", UInt, hVolume)
}