I just happened to do a Windows Update check while I had an SxS card plugged in. Windows update suggested an optional update on my SxS card. What's that all about? I declined the update because I had live data on the card that I was preparing to download from the card.

I would imagine Windows was going to re-format the card to the NTFS file structure, I believe SXS uses the FAT32 structure.

You were wise to leave the update alone, although I am sure you would have had a re-format warning first.

What card do you have? Doubt this is the issue, but reading your post reminded me of it....

This is firmware update for SxS-1 Memory Cards (MODEL: SBS-32G1). In recent months it has been discovered that, on very rare occasions, while shooting with an SxS-1 memory card, the camera will stop recording before the card is full and a “media error” message will appear on the camera LCD. This latest firmware update provides protection from this occurrence. Sony recommends that you contact us at your earliest convenience to have your SxS-1 memory cards updated with this important firmware.
NOTE: SxS PRO memory cards (MODELS: SBP-8, SBP-16, SBP-32) are NOT affected.

http://pro.sony.com/bbsccms/static/files/mkt/recmedia/sbs32g1firmwareupdate0810.pdf

The same happened to me on my Win7 64bit today. Windows updater suggests an optional update from Sony Corporation for SxS memory cards. I had a 32GB SxS Pro inside of my PCI card reader.

I did not agree the update till now but I would assume it does not do anything to the card itself, at least no formatting. It looks like if it would be just a normal driver update for Windows. In the same moment it suggested an optional update for my nVidia graphics card.

Details for the update:
Sony Corporation - Storage - SxS Memory Card
Downloadsize: 91 KB

Updatetype: Optional

Sony Corporation Storage software update released in July, 2010
http://winqual.microsoft.com/support/?driverid=20376202