library has an access door这个access door 是？

在查询I/O error 不知下的 access door 是什么东西？
ANR8418E Command: An I/O error occurred
while accessing library library name.
.
Explanation: An I/O error occurs while accessing  the designated library.
.
System action: The operation fails.
.
User response: Ensure that the DEVICE parameter
associated with the library is identified correctly
in the DEFINE PATH command, and that the library
is currently powered on and ready. If the library
has an access door, make sure it is closed. After
checking and correcting these potential problems,
retry the operation.

access door 是指库门？