Windows 7 x64 seems to have a long-standing and serious bug in the USB disk I/O system. Few users notice this unless the file has some sort of built-in error detection capabilities. After getting quite a few corrupted files on my perfectly working USB disks and flash drives over the years, I searched various forums for similar reports. Such Win7 problems go back to day one for Win7, for 64-bit installations. It does not seem to depend on the h/w at all, and it is not a USB3 issue. Here's a summary. 1. Corruption occurs when writing to a USB storage device, but not when reading from a device. 2. The corruption is usually not reproducible. It is quite flaky and random. I suspect this is a software/hardware timing issue with buffers, rather than a logical error in the Win7 operating system. 3. I usually store offline files in archives (zip, 7z, etc) or compute checksums for newly written large files. This is how I became aware of the problem. Corrupted files may be of any size. 4. The problem is not related to any updates or service packs. Seems to be confined to Win7 x64. 5. As shown in the next post, the error is an absence data in whole sectors of the o/p file. 6. One user has reported that switching the USB caching policy from write-thu (quick disconnect) to write (performance) will cure the problem. ======================= A specific example I wrote a 1GB file from my laptop (Ausus EM642G) to a 750GB WD USB disk. The disk was new and fully zeroed (0x00 in every byte) before formatting. A SHA1 check showed that the new file on the external disk was not the same as the original one on the C-drive desktop. Using unix tools the following were results found. 1. The new file was the correct length. The first 555MB (55% of 1GB) was identical to the original file. There were then 40 consective sectors in the corrupt file, that had zeros in them. It was exactly 40 of 512byte sectors. The next few megabytes were correct. Futher errors were all in exact groups of 512byte sectors. 2. The zero sectors were mostly likely due to the free space being all zeroed. The problem was that no data at all was written to them, rather than bad data. 3. When pasting in Win7 , the progress bar went to about the 50% mark instantly, and then filled up to the 100% mark at normal speed. 4. Looks like a buffer flushing problem. There are multiple s/w buffers for a single i/o opertion. Disk space is allocated correctly, but the buffers never get written. -------------------------- This is a very serious problem, and MS needs to admit it, and to fix it. The average user is mostly unaware of this corruption. When they do find this problem, the USB device is always blamed, and junked.