Opening external hard drive photos

First, how is your SSD drive formatted? To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:

Move your Photos library to save space on your Mac - Apple Support

Photos uses a database, sort of like an Excel Spreadsheet, with columns of information about rows of pictures. Each new version has more information and more columns, so one version can't understand the database in another. When a Photos database is loaded into a newer OS, the database is converted, with the new columns put in and new scans done to fill those columns. So there can never be "going back." The only way to access different versions of Photos Libraries is through iCloud Photos, where there's enough room and power to handle multiple databases.

It's not clear to me what you're trying to do-- Are you trying to open a Monterey Photos Library with a newer OS, or are you trying to open an older Library in Monterey? Opening an older Library should work, unless it's maybe way too old. Do you have the Mac on which the original Photos Library was created?

Let us know about the OSs and the format of your drive…

I have a Case-Sensitive drive that I made because I was collecting old backups that had pictures like DSC_0023.jpg and also DSC_0023.JPG, and I decided to keep them all and sort them out later. I still hate using that drive! I'm pretty sure, that Case-Sensitive is a problem for Photos. I imagine Photos searching frantically for the picture Houseboat.jpg when it's listed as houseboat.JPG.

If on that drive you have two copies of pictures with the same name except for upper and lower case, then it may be painful to switch--for all the reasons that you have to. You may have to buy another disk drive and transfer the Photos Library over. In the Photos Library, pictures aren't all kept in the same folder, so .jpg in one won't get confused with .JPG in another, and you may be OK. You can buy a 500 GB mechanical drive for under $30, I think. TB SSDs are faster, smaller, better all-around, and more expensive.

I think you can get the pictures, but if they are in a Photos Library, don't mess with them until you have them in a trustworthy environment.

On a Mac, locate the hard drive. I list the drives in the Sidebar of Finder Windows. Sometimes you see it on the Desktop. Then select the drive, and open the Info Window ( ⌘-i ) and you should see something like this:

Here you can see that this drive is formatted APFS.

Many drives are formatted in ExFAT, a format that Windows and Macs can both read. But it's old and not as versatile as the two Apple formats I had mentioned. APFS is great in new solid state drives, but Mac OS Extended (Journaled) is common in spinning mechanical drives.

If you want the pictures in a Photos Library-- we can get them!

Any drive (that works, of course) can be formatted (or re-formatted) for Mac. Use the app Disk Utility. I just start typing into the Spotlight field

and it fills in the rest.

You choose Erase

and, either Mac OS Extended (Journaled) or APFS. Don't use Case Sensitive. If the Drive is Solid State, definitely choose APFS.