Files transfer stops "because some data can’t be read or written?"

I've never known an standard OS interface to work that way which is why I suggested Carbon Copy Cloner since it will skip the bad files and also make a log entry so you can know which files were skipped.

I recommend that you attempt to use DriveDx to check the health of the drive since that those symptoms are a classic sign of a failing drive. If DriveDx reports and "Failing" or "Warnings" on a hard drive, then it inidicates the drive is either failing or worn out respectively (SSDs are a bit different). I will be happy to review the DriveDx report if you post it here using the "Additional Text" icon which looks like a piece of paper.

Definitely make sure to have a backup of all important & unique data on all of your external media. Don't ever rely on just one copy of your important files or next time you could lose all the files.

Try running Disk Utility First Aid on both external drives.

Are both drives connected directly to the computer? If not, then the hub or adapter could be defective. If the drives are both connected directly to the computer, then you can try using a powered USB3 hub with UASP support.

My guess is one of the external drives is failing. You can attempt to check the health of the external drives by running DriveDx. You will need to install a special USB driver so that DriveDx can attempt to access the health information on the external drives. However, some external drive enclosures do not allow the necessary communication even with the special USB driver. Post the report for both external drives here using the "Additional Text" icon which looks like a piece of paper. Usually any "Failing" or "Warnings" mean the hard drive is bad or worn out respectively.

<<How do I keep the whole process from stopping due to only one bad photo?>>

Unfortunately, if it copied all-but-a-handful, Finder has no way to tell you which files did not copy.

So Instead, it stops. Your mission is to divide your files into two groups, copy each group, and keep diving in half the failed group until you find the defective file.

Use Carbon Copy Cloner to copy the files. It keeps a log of the files it doesn't copy plus it will also prompt you if you want to skip the errors. Sometimes this will allow you to copy the files that are still readable, but if you encounter too many errors and the copy moves along extremely slowly, then I would abort it since it likely indicates a failing hard drive.

Your symptoms really sounds like you have a failing hard drive (classic signs). The more you use a failing the drive the worse the failure can become where even a professional data recovery service will be unable to recover any files.