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After installing Vista SP1 you may experience a lot of failures. One of the most irritating one which is strictly related to the famous USB “unknown device” issue, is the Readyboost feature which stops working all of a sudden. Basically the USB key is not recognized anymore. To fix it, follow this trick:

1. Click Start.
2. Click Run…
3. Type Regedit.
4. Find this key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\EMDMgmt
5. Remove all the sub values. They looks like folders. They should look like this: _??_USBSTOR#Disk&Ven_Multi&Prod_Flash_Reader&Rev_1.00….
6. Re-insert the USB key.
7. Now, reopen Regedit and change the following values in the memory stick settings Vista has just created: “CacheSizeInMB”=dword:000003c0 “CacheStatus”=dword:00000001 “DeviceStatus”=dword:00000002
8. Close Regedit, right click on the USB Key on My Computer, Set the cache of your ReadyBoost
9. Done!

Source: forums.microsoft.com