Radio Shack TRS-80 Model II Reference Manual page 77

Library Commands
I
Swap Diskettes
You should execute this command immediately after swapping diskettes. It tells
TRSDOS
to read the diskette !D's on all drives in the system.
Note: Do not swap diskettes while a file is open.
See
VERIFY
for related information.
Example
I
tells the System you have changed one of the diskettes.
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