Include subfolders setting when synchronizing a USB HD

Hello.

Im using LR3.

Recently some of my photos went missing from my Library, but they are still on the external Hard Drive.

I am trying to resynchronize the entire drive, but the Import or synchronize window tells me that "include Subfolders" is set to off, without giving me a way to switch it on.

How do I switch it back to on, as I recall it was set before?

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Andrzej
http://lowflyingbananas.zenfolio.com/