[Milhouse]

I was thinking the same as you. However, here's the weird thing: If I take the hard drive offline again after the malformed urls have been created, the extrathumbs remain cached and are displayed.

The only way I can think the skin is doing this is by referencing a local cache of filename data, maybe common cache populated while the source is online. I could be wrong though, however the simplest method is to just lookup the directory and see if it exists then use whatever files are inside. More complicated methods might allow some offline capability, I guess.

It's almost like the skin doesn't like how texturecache.py cached the extrathumbs, so it had to redo the caching...or something.

That's because the skin has a bug and is constructing the extrathumbs filename incorrectly - it's redoing the caching because it's not using the correct filename (url) that is already cached.

Extrafanart does not show up at all when the hard drive is offline no matter what.

Odd that it doesn't work the same way as extrathumbs, and it also doesn't seem to suffer from the malformed urls either. Probably best to ask the skin authour what is going on, and would be nice if they fixed the malformed url - that seems to happen quite often, unfortunately.