The problem could be transmission from the camera card to your computer ... any piece of hardware in the chain could be the culprit ... camera card, card reader or USB cable, hard disk (the original hard disk or the new external ones) or even computer memory. Try transferring the photos using different hardware, or even a different computer.
Thank you for your reply.
Camera card: No, I am using more than one and the files show up fine in LR when stored on the internal HD. The same goes for the card reader - it is built into my computer and if it was problematic, images would be corrupted regardless of where they are stored. USB cable - I tried three different cables with 2 external HDs - same problem. The hard disk - again, images are fine when stored internally, but there is no space left.
Now, my concern is: Are the images actually corrupted on those external drives (hard to imagine that they would be, as I bought two different HDs), OR do they just LOOK corrupted in LR because of some weird computer memory problem?
It sounds like you haven't ruled out memory as the cause of the problem.
I don't understand your logic for ruling out the card reader, but maybe you haven't really explained how the photos got onto the external HD. In other words, what was the workflow that resulted in the same photo from the same original on the camera card on the three different disks?
Thank you for replying. That is right, I am quite suspicious of memory - my laptop is full to the brim and I suspect that this may affect available memory and cause some problems in LR. Unfortunately, I don't know enough about computers to be certain.
The card reader - quite simple: the card reader is the same, whether I save the files to the internal or external drives. I put the SD card in it, then copy the files and paste them where I want them. When done, I then open the LR and synchronize the folder. When I copy and paste files to the internal drive, the images look fine in LR; when I copy and paste them into the external drive, 80% of them come up visually corrupted in LR. Since the same card reader is used in both cases, and the files are OK when saved internally, I am assuming it is not the card reader.
So, basically, when I became aware of the problem, I tried copying and saving the same file both internally and externally from the card reader, in order to verify what was going on. That is how the same file (more than one, actually) ended up on three different drives.
Right now, my instinct is to copy everything to an external drive and clear some space on the internal one. I am very apprehensive though - what if something is causing file corruption on the external drives - in that case I will be corrupting everything when I migrate the files. Hence my previous question - are the files actually corrupted, or do they simply appear corrupted in LR because of memory issues? As I said, I don't know enough about this to be certain and do not know what to do.
Have you tried copying back one of the apparently corrupted files to your laptop's internal drive, renaming it and then re-importing into LR? Or using an alternative program to view the apparently corrupted files? This might at least let you know if the files are really corrupted. Also how are you copying the files to the external drive (ie: letting LR do it or doing it yourself and then importing into LR)?
As you have already surmised and DJ suggested memory is often the cause of these problems. How much memory do you have? (Memory, not free internal disk space). I know LR is an intensive user of memory but can't comment as to whether it's more memory intensive to access an external drive than an internal one (though it wouldn't surprise me if it is, might depend on the connection type too). What type are the external drives (USB etc) ? Has it always been the same hardware port on the Laptop that you've connected the external drives to?
Sorry a lot of questions but they might shed some light....