To rebuild the thumbnails:
Back Up and try rebuild the library: hold down the command and option (or alt) keys while launching iPhoto. Use the resulting dialogue to rebuild. Include the option to rebuild the thumbnails. It can take 3 or 4 goes for this to work
Yes, back up first. You need an up-to-date back up of all your important data all the time.
To migrate from iPhoto: That's what File -> Export is for...
That said: you photos are no safer in folders in the finder than they are in iPhoto. (After all, Folders in the FInder is exactly how iPhoto saves them. An up-to-date backup is protection against problems. Giving up Photo protects you against... well nothing.
Thanks TD. Unfortunately I don't have my EHD with me (2TB EHD is too difficult to travel with) so will have to wait until Monday when return home to backup and then try to rebuild. I think what you are saying is that the backup won't override the back up time machine made before I left if I backup to the same EHD on Monday (in case the backup I do on Monday is of a faulty iphoto) and I can resort to the backup I made before I left if I need to. btw i hate to think how long a rebuild of 20000 photos & videos (RAW and jpeg) will take!
Also, I've just noticed that the thumbnails that are missing seem to be of those images that I have duplicates of that I didn't realise I had (seems I downloaded photos from my iphone and old computer that were duplicates when I first got this macbook). Have you heard of that before? Maybe all I need to do is delete the duplicates and that will fix the missing thumbnail problem and iphoto crashing? (Don't know why its taken two years before it caused problems though).
If so, the question is then which duplicate do I delete - the one with the missing thumbnail, or the other one? The one with the missing thumbnail has a normal file number (eg. IMG_0778) whereas the one that has the thumbnail showing has a strange file number (eg. IMG_0778-1). Have you ever come across that before?
If my photos are safe in iphoto, why is iphoto crashing and why is it doing funny things to the thumbnails? If they were just in normal folders in Finder I would know if the file had something wrong with it just by opening it, in iphoto I can't do that - I have no idea where the file is . . . (other than somewhere in 'Pictures' in a library in a 'package' that everyone tells me I'm not allowed to touch so I'm too scared to even look in it!) Also a normal folder system surely would have asked me when I was downloading duplicate files whether I wanted to replace the original rather than creating another copy with another filename like it appears iphoto did? (My other issue which may have a solution but I haven't checked apple support communities yet is that every time I open a photo in an external editor iphoto makes a duplicate of it even if i don't make any changes - but thats for another day). I would love you to convince me to keep iphoto as I love all the other features but this experience really has shaken me.
If my photos are safe in iphoto, why is iphoto crashing and why is it doing funny things to the thumbnails?
This has nothing to do with the actual photos, though, They are - by your own description - safe and sound. The file has nothing wrong with it.
You can see any of your master files simply by using the Show File command.
The important thing about the library is that you make no changes with the Finder - don't move anything, don't rename anything. So, look inside:
Go to your Pictures Folder and find the iPhoto Library there. Right (or Control-) Click on the icon and select 'Show Package Contents'. A finder window will open with the Library exposed. Your original files are in the Folder called Originals - or in later versions, Masters.
Is this crashing an ongoing problem? From your first post it sounded like a once-off?
Thanks again. I dared to peek inside but it didn't help me much! I'm not sure of the system of filing it has created. eg. it starts with a 2011 folder but I have images from before then. I can't find the files it has renamed with a -1 or -1-2 after the name.
crashing is not an ongoing problem, my macbook crashed twice before but not sure it was iphoto - and so I updated my macbook to 1TB harddrive and 8mb of ram. After that I still got the 'wheel of death' often when using iphoto, but this was the first time it crashed since then. And the very first time thumbnails went missing.
Its after midnight here so I'll check in again in the morning. thanks again for your time.