As a trouble shooting procedure, check out the following Apple link re storing a Photos library on an external drive:
https://support.apple.com/en-us/HT201517
To prevent loss of data, Apple recommends against storing a Photos library on a thumb drive. If you try it, the thumb drive, or any external drive used for Photos library storage, should be formatted Mac OS Extended (Journaled). Be aware that reformatting a drive, if you need to do that, erases all data on it. So first back up elsewhere any data that you want to keep.
As for creating files for the photos that won't export, you might first check your photos library to see if the photos display properly there. If they do, then they are intact and there is nothing really to recreate. However, if the photos are missing or corrupted in your Photos library you might try doing a library repair by opening Photos while holding down the Option and Command keys and selecting to repair the library in the box that appears.
-- Rich