Recently, I've been having a weird problem with Picasa. Sometimes, after modifying an image, Picasa would refuse to save it. It claimed a "disk error" that might mean the disk was full or read-only. Well, that's not likely. I have 349 GB of open, writable space on my hard drive.
So I did some Google searching, and it appears that there are at least 6 different things that can cause the problem. A lingering 'export to' address pointing to a now-disconnected external disk can be the issue, as can extremely large (greater than 10,000 x 10,000 pixels) images. But none were causing mine.
Then I found this discussion on the GIMP forums, where "acmespaceship" found the answer. The new Export function in GIMP 2.6 is normally set to add XMP metadata. For some reason, Picasa doesn't play nicely with this XMP data.
So, the solution: when exporting, click the (+) button to see advanced options, and uncheck "Save XMP data". Picasa will stop complaining.