Unread sectors in Map mode

[kikiki]

There are black colored sectors in Map mode, they are filled with 0xFFs, they are defined as "unread sectors" by PC-3000.
Why they defined as "unread sectors", did the reader skipped reading them for a reason?

Thanks.

[pclab]

What color are those sectors? Red, yellow, black...

[pcimage]

Black sectors are unreadable sectors, usually due to being physically bad or sometimes due to other issues (e.g bad translator etc...)