Added: I think we are living in different time zones, sorry for not responding earlier, but your last question arrived at 3:52 AM!
it plays well in fullscreen, although sometimes the videos are a bit delayed. My iphoto library is on my computer. Not on an external harddrive.
Is it a second drive in your computer, or is it on the system drive? For the system drive the formatiing is probably correct, since MacOS X needs to be installed on a correctly formatted disk.
If it is a second internal drive, check the formatting as described above. If the format is wrong(MS-DOS (FAT)) or something, you need to backup all data on the drive and refrmat it with Disk Utility.
I just made a mini slideshow with about 5 pictures, no videos & I get the same message. 😟
Try to repair your iPhoto Library - repair the permissions and repair the database.
How to will depend on your iPhoto version - this document describes how to repair for different iPhoto versions, see: iPhoto 6 and later: Rebuilding the iPhoto library
What is your iPhoto Version and your MacOS X version?
What is the format of your photos? Raw or Jpeg?