From: Sam Mitchell 
This seems to work for me:
constant
    PATHLEN =3D 80
string kill_this_history[PATHLEN]
string pick_footer[] =3D "{Right}-Next Drive {Left}-Prev Drive"
integer proc ChangeDrive(integer delta)
    string drive[1]
    drive =3D Chr(Asc(Query(PickFilePath)) + delta)
    if FileExists(drive + ":\*.*")
        kill_this_history =3D GetDir(drive)
        PushKey()
        PushKeyStr(kill_this_history)
        PushKey()
        if FindHistoryStr(kill_this_history, Query(CurrHistoryList))
            kill_this_history =3D ""
        endif
        return (TRUE)
    endif
    return (FALSE)
end
integer proc NextDrive()
    return (ChangeDrive(1))
end
integer proc PrevDrive()
    return (ChangeDrive(-1))
end
keydef pick_keys
           if not NextDrive() Alarm() endif
            if not PrevDrive() Alarm () endif
end
proc PickDirStartup()
    if Length(kill_this_history)
        DelHistoryStr(Query(CurrHistoryList), 1)
        kill_this_history =3D ""
    endif
    Enable(pick_keys)
    WindowFooter(pick_footer)
end
proc WhenLoaded()
    Hook(_PICKFILE_STARTUP_, PickDirStartup)
end
---
---------------