I spent 3 hours this morning attempting to figure out where to start looking. The problem shows up as a difference between files which are almost certainly identical - except in one copy which I am confident is different all of the time by the specified byte (caused at the time the copy was made). Rather than attempting to hide that this is current PC hardware, it is probably best to list: Hardware: Intel E8200 CPU (2.66 GHz), 2 * 2 GB memory in ASUS P5B motherboard with 2 * Seagate 320 GB SATA II hard drives Software: Windows XP with probably SP3 The problem shows up between 5% to 10% of the time with very large files of greater than 1/2 GB to 2 GB in total size. The command: COPY /B D:*.GHO G:*.GHx was used to create 4 files each time with x = A,B,C,D,E FC /B D:*.GHO G:*.GHx is used to compare the files 4 at a time. It takes about 2 1/2 minutes to compare the group of 4 files for a total of 5 GB being compared with 5 GB (which is why I said it must be current hardware). The result often (between 5% and 10% of the time) shows a single difference between the files at byte xxxxx35A (the xxxxx is random but the last 3 characters of the address are always the same) with the two hex characters for each byte being different by 1 bit: e.g. 22 vs 32 about 25% of the time with an extra bit 4 e.g. 94 vs 84 about 75% of the time with a missing bit 4 I probably performed between 75 and 150 FC commands during which about 200 comparisons were made. I collected about 15 cases when a difference was noted. I can also perform an MD5 valuation of each file. So long as the available memory is exceeded (relative to the size of the previous files that were just tested), the MD5 valuation takes about 40 seconds for a 2 GB file and the disk sense light is on the whole time. If the same file is repeated for the MD5 valuation, the disk sense light is always off and the MD5 valuation takes about 9 seconds. The MD5 valuation is probably incorrect between 5% and 10% of the time. Since repeating the MD5 valuation on a different copy of the same file provides a cross check, when what seems like an incorrect MD5 valuation appears, checking with a different copy will almost always (19 times out of 20) show the correct MD5 valuation, then when the MD5 valuation is redone on the same file, the probably correct MD5 valuation appears, i.e. different from the just previous MD5 valuation on the same file. However, if the MD5 valuation is done again without flushing the cached copy, then the same memory contents seem to be used each subsequent time yielding the same (probably incorrect if that was the situation) MD5 valuation from the last time that the disk drive was actually read. My assumption is that as the (now incorrect) byte was read into memory, one of the bytes of RAM (possibly the same one each time) gets an extra bit set or misses one of the bits being set - always that same bit 4 of course. Other reasons may also have caused the byte in RAM to be incorrect. However, once incorrect, it seems to stay the same until it is modified again. So it does not seem to be a problem with reading RAM. So (FINALLY) this is my question: If the cached copy (probably incorrect) of a hard drive file (which is used to perform the MD5 valuation) stays the same, what is the probability that it is a problem with RAM memory as opposed to a hard drive or controller error? Since the error suggests that it is a single bit causing the problem (the same bit seems to be either on or off), my intuition would seem to suggest that the RAM memory has a problem. However, since my hardware experience is minimal, I am asking for suggestions as to what can be done and the order they should be attempted: (a) Replace the RAM memory (b) Replace the disk drive(s) (c) Replace the motherboard with the controller problem (d) Something else I have not thought of I know that Tony Duell would want to fix the component, but I doubt that is even possible in this case. Does anyone have any different suggestions? Thank you for any help and please suggest any different tests that might help to locate or better identify the cause of the problem. Finally, just to make sure, does this problem constitute a serious difficulty which should (must?) be fixed before any actual use is made of this new system? Sincerely yours, Jerome Fine