Edit 2: I've reinstalled Windows and the problem remains, so I guess WU is not the problem. I'll leave the rest of the post as the problem remains.
After the necessary Windows Update on Thursday, one of my hard drives showed extremely slow write access (500kb/s, compared 40mb/s when reading). I have another Samsung drive of the same model on the same motherboard port which works fine.
The drive tested fine and works flawlessly in Knoppix (normal write rates). I made sure that Write Cache is enabled (cannot be disabled anyway). I installed all the latest drivers. SFC does not report any problems. No viruses or trojans were found. Process Explorer does not show any unusual activity.
I run Windows 7 32-bit Professional. Re-installed it two weeks ago. Never had a problem with the system before. I restored the system to the state of before the update installation, so I am not sure the two are connected. It's just curious that it's only one drive and that only write access is slow and that it works under Knoppix.
Edit: I found out that write access gets slower over time. I wrote a small tool to copy a large file in chunks of 512KB and at first the write rate is normal (40mb/s) and then it gets slower over time:
94883kb/s or 92mb/s. 13846kb/s or 13mb/s. 13784kb/s or 13mb/s. 13721kb/s or 13mb/s. 13658kb/s or 13mb/s. 12019kb/s or 11mb/s. 10237kb/s or 9mb/s. 7521kb/s or 7mb/s. 7502kb/s or 7mb/s. 7484kb/s or 7mb/s. 6963kb/s or 6mb/s. 6511kb/s or 6mb/s. 6114kb/s or 5mb/s.
Down to 1500kb/s. I have no idea how reliable this tool is, but even when copying normally with windows tools it starts reporting a normal write rate and then it slows down. When I copy a lot of small files, the first ones get copied fast and then it gets slower with every file. This has to be connected to write caching, but why would it be faster under Knoppix?