Question: If I burn the CD, can new files not be copied onto it?  I burned one just to test it, but when I tried to copy another file onto it, it said that it couldn’t be altered. Answer: CDs and DVDs are permanent. Once you burn a CD or a DVD you cannot delete [...]

When your iPhoto library starts getting very large, or you transfer it from one computer to another (and these are just examples, there are other potential causes) you may need to rebuild your thumbnail cache. You can otherwise be prone to uncharacteristic sluggishness while scrolling, weird thumbnail duplication, or they might just be missing altogether. [...]