moved library to external hd but thumbnails remain

How did you "move" theses photos and movies?

The correct method is

Moving the iPhoto library is safe and simple - quit iPhoto and drag the iPhoto library intact as a single entity to the external drive - depress the option key and launch iPhoto using the "select library" option to point to the new location on the external drive - fully test it and then trash the old library on the internal drive (test one more time prior to emptying the trash)

And be sure that the External drive is formatted Mac OS extended (journaled) (iPhoto does not work with drives with other formats) and that it is always available prior to launching iPhoto

And backup soon and often - having your iPhoto library on an external drive is not a backup and if you are using Time Machine you need to check and be sure that TM is backing up your external drive

The ! indicates a disconnect between the Thumbnail and the original

this can be caused by two things

1 - User action deleting items from inside the iPhoto library - can be directly - can be from the All images list (never delete from there) - can be from running cleanup software like MacKeeer, CleanMac2 or other dangerous, defective software that improperly deletes or changes things in the iPhoto library - if you have done any of these things you must restore your backup from before you did them and damaged your iPhoto library - and I recommend getting rid of any "clean up software" ASAP (although being essentially MalWear it is not easy)

2 - corruption of the iPhoto library - If it is not #1 then Back up your iPhoto library, Depress and hold the option (alt) and command keys and launch iPhoto - rebuild your database

LN

Something is wrong with what you are telling us - if you put the iPhoto library in the trash and emptied it then you have no thumbnails or anything else remaining from it - you either did something different or you have another iPhoto library floating around - iphoto only displays things in the current iphoto library

As to backing up - Yes TM will backup external drives - simply open the options in the TM preferences and remove the EHD from the excluded drive list

And I have no idea what you mean by

I would love to keep backing them all up on time machine, but how? if once it fills it isn't deleting older backups?

Yes that is how TM works (as do most if not all backup systems - a backup is NOT an archive - it is a backup of what you have - and the case of TM there is some limited archival capability but no long term archival capability)

LN

for TM questions post in the forum for your OS

and see - https://www.apple.com/support/timemachine/ and http://pondini.org/TM/FAQ.html and http://pondini.org/TM/Troubleshooting.html

LN

If you want to start over with a new library then do so. Delete the old library as it has remnants of old database files, etc., that you don't want hanging around. You want to start with a clean library so launch iPhoto with the Option key held down and click on the New Library button.