Viewing Pics from external HD created on earlier system version on a Computer with more Current Software???

No, select the drive, hit cmd-i - look at the top circled section. What do you see?

It must be formatted APFS or MacOS extended (journaled) (preferred for spinning drives). Not case sensitive. If you have not formatted it after purchase it will most likely be ExFat - which is not compatible.

It Must not be (or have been) used for time machine. (A time machine drive can be used if re-formatted)

It must have the 'ignore ownership" checkbox ticked.

It must be directly connected (No ethernet or wifi)

See

Move your Photos library to save space on your Mac – Apple Support (UK)

If a library was created by an earlier version of Photos it will be converted to the current version the first time it is open with the new Photos app. There will be no loss of photos.

However, it's good practice to have a full and recent backup of all user data on a continual basis.

What format is the external drive?

If the external drive is correctly formatted, either HFS+ (rotational drive) or APFS (SSD), then try opening the library with the Option+Command keys held down and try to repair the library.