CB-Images wrote:
Hi Guys
I'm just arriving on the end of this discussion, I have to admit most of it is way over my head although I follow the principles.
We've tried to weave simplified summaries in with the highly technical stuff so you can get the gist without understanding everything.
CB-Images wrote:
after going through my Library of downloaded images everything appeared OK. I opened a few images into the Develop module, processed them, exported to Photoshop - again everything was fine.
This means your problem is not due to card, nor card reader, nor cable, nor USB port... i.e. images not corrupt after import and after raw rendering (raw rendering is done in the background, after initial import may appear ok, based on embedded jpeg previews), were not corrupted upon transfer.
CB-Images wrote:
Yesterday when I opened up the Library containing the same RAW (CR2) images the thumbnails appeared to be redrawn as the thumbnails came up and then moments later the images changed slightly, I assumed Lightroom was applying the tone mapping corrections I had made...
I assume so too: - sounds like (lib) previews were being recreated, in response to develop setting changes.
CB-Images wrote:
I was initially worried but then as I had a back-up and it wasn't a particularly important file I thought - 1 problem in 3 years of using Lightroom isn't too bad.
Indeed, many of us will go a lifetime without ever experiencing such corruption, which is why whether any particular user has or has not experienced it may not seem relevant to others who are experiencing it.
CB-Images wrote:
After reading through this discussion I opened Canon's DPP software and the same RAW file appears OK in DPP.
*Iff* Canon's DPP was showing you a valid raw rendering (i.e. we're *not* talking about the thumbnail, which can show a valid image despite having a corrupt file, since it only needs the embedded jpeg to be ok for that, which is a small part of the file compared to the raw data - I can't tell from your statement which kind of "appears OK" we're talking about), then file is not generally corrupt on disk. I say "generally" because it's possible if disk is marginal for it to be read ok one time, then not the next.
CB-Images wrote:
Today I have been processing some star trail images I took 2 nights ago.
These images have been imported into Lightroom and basic corrections carried out to the RAW files without problems.
I tried opening all 180 images into Photoshop for merging but Photoshop ground to a halt under the strain so I decided to open them in batches of 20. I selected 20 images in Lightroom, used 'Open As Layers in Photoshop', processed them and then on my 6th visit back to Lightroom one of the next batch of 20 images suddenly became corrupted as I selected it. All the thumbnails appeared to be updated/redrawn again as they all changed slightly everytime I returned to Lightroom. Lightroom also seems to have created a .tif file of this corrupted RAW file.
I couldn't tell - is tif file ok? or does it look wonky? - it would be helpful to show screenshots of wonkiness (corruption) - hardware failures have a telltale appearance - discolored rectangles and stripes, like the one originally posted in this thread. One thing to note: if file appears wonky sometimes but not other times, or not in other software, then the problem (assuming hardware failure for the moment) is occuring upon reading the image. In other words, it is not corrupt on the disk.
CB-Images wrote:
I have tried saving it with a different name and importing it into Lightroom, it appears OK as a thumbnail but when I try to open it, it is corrupted.
This is the case I was talking about previously - embedded preview can be read OK, but not the raw data: not that unusual, when this sort of thing is happening.
CB-Images wrote:
As I understand it, this could be due to faulty RAM, a failing HDD, a faulty card reader or cable.
1) Faulty card reader - fairly inexpensive, so I will try replacing it.
Logically wouldn't I be seeing more corrupt files if the Card Reader was responsible and not just 1 out of the 200 - 300 I have downloaded on each of the last few days?
Your problem is not from card reader nor cable... - images that are completely OK after import (e.g. can be processed in develop module / Photoshop / ACR), were transfered OK - forget the reader...
If you had a faulty card reader, it would be entirely possible for the failure to be intermittent, occasional... - do not assume... - they can be borderline / thresholdy or totally wonked.
CB-Images wrote:
2) Faulty RAM - someone suggested trying Memtest86, I looked at the web site and it looks far too complicated, I'm a photographer not an IT consultant.
Is there any easier way to check RAM or an idiots guide to using something like Memtest86 on a Mac?
Please report back after you succeed or fail with the bootable Memtest86 CD. As DdeGannes can attest, passing this test does not necessarily mean you have no ram problems, however failing it does mean you have bad ram, without a doubt. - usually bad ram will fail this test, but not always.
CB-Images wrote:
3) Replacing the HDD - I know HDD's can fail at any time but wouldn't I be getting all sorts of other problems if the HDD was begining to fail?
HDD's usually (read: not always) fail slowly at first, then go to hell in a bucket fairly shortly thereafter. So again - make no assumptions: it is entirely possible for hard disk to have bad spots that only affect some files... - disk can fail, or supporting electronics can fail, more often the former. Even when the electronics is failing it's possible for it to only have problems with some areas, not the entire disk - granted is is more often a problem disk-wide in the case of failing electronics (control circuitry).
CB-Images wrote:
3) Replacing the HDD - I know HDD's can fail at any time but wouldn't I be getting all sorts of other problems if the HDD was begining to fail?
This will be a pain to change as I believe I have to take my iMac to an Apple store, a friend had his HDD replaced and it took over a week but I want to be sure that this is where the problem lies before I go down this route.
I would like to discount the other 2 possible problem areas first.
It may be a pain to replace the HDD but it's *not* a pain to check it - I recommend doing that (checking it) ASAP.
CB-Images wrote:
I do think having a Forum like this is excellent to help us all find ways of solving the problems we all encounter from time to time, even if they do get too technical for the average person now and again.
Please report back. Much of what those of us who are not having problems learn, comes from the findings of those of you who are having problems... 
Maybe somebody else will see something I missed. Generally though: problems can occur writing or reading. If OK sometimes, then problem is reading, if OK never, problem may have occurred while writing.
Good luck,
Standing by,
Rob