iPhoto Library not recognised by Photo

What is the make, model and size of the "flash drive"? How is it formatted? It must be formatted either APFS (for SSD) or OS X Extended (journaled) for HDD.

If it's not formatted as one of the two formats I mentioned try to copy the library to a volume that is formatted correctly and try to open it there.

Hey 1saltyseadog ,

We'd recommend checking out this article as it provides information that may help resolve your issue: Create additional photo libraries in Photos on Mac - Apple Support

"Designate a library as the System Photo Library

You can switch to another library and designate it as the System Photo Library. 

1. In the Photos app  on your Mac, choose Photos > Settings, then click General.
2. To make the current library the System Photo Library, click Use as System Photo Library. 

Important: If you switch to another library, Photos automatically turns off iCloud Photos and Shared Albums. You can turn them on again in the iCloud pane of Photos settings. When you turn on iCloud Photos after designating a new System Photo Library, the photos stored in iCloud are merged with those in the new System Photo Library. After the content from the new System Photo Library is uploaded to iCloud, the libraries can’t be unmerged, so consider carefully before changing your System Photo Library."

You may also want to try moving your library to a different external drive or to the internal HD on your Mac to see if the issue is specific to the flash drive.

If you need further assistance, contact Apple Support here: Contact - Official Apple Support

Thanks!