cannot copy files from external drive, but can read though

If CCC is unable to copy your files, then the drive is producing too many errors for any standard macOS utility. Using the command line utility GNU ddrescue to perform a bit for bit clone of the failing drive as suggested by @Tony T1 is the only way to possibly recover the files on that drive except for using a professional data recovery service such as Drive Savers or Ontrack. Both vendors provide free estimates and both are recommended by Apple.

To use ddrescue requires another drive of equal or larger size.

You need to choose wisely which way you attempt to recover your data from a failing drive since you usually only get one chance.

I recommend booting from a Knoppix Linux USB drive since it includes GNU ddrescue and Linux is better able to handle the errors produced by a failing hard drive. You can use the downloaded Knoppix .iso file as a source for Etcher to create the bootable Knoppix USB drive. Option Boot the USB drive and select the orange icon labeled "EFI".

You can also install GNU ddrescue by using Homebrew. If you use macOS to clone the failing drive using ddrescue, then you will first need to "unmount" all volumes on the failing drive. "Unmounting" the drive is different than "ejecting" the drive using the Finder. You will need to use the Terminal app and the command line to "unmount" the drive. Begin typing the following in the Terminal:

diskutil  unmount  

Then drag & drop the external drive volume from the Finder window to the Terminal window to auto-fill the correct path to the volume of the failing drive. Do this for each mounted volume on the failing drive. You don't need to do this step if booted from a Knoppix Linux drive.

You need to be extremely careful when using GNU ddrescue as it is very easy to make a mistake with the source & destinations for the command. If you make a mistake you may overwrite the data you are trying to recover.

Homebrew:

https://brew.sh/

Install GNU ddrescue using Homebrew:

https://formulae.brew.sh/formula/ddrescue

GNU ddrescue manual:

https://www.gnu.org/software/ddrescue/

Sounds like there may be a file system issue or perhaps even a failing hard drive.

Try running Disk Utility First Aid on the external drive.

Try using Carbon Copy Cloner (CCC) to copy your files instead. At least with CCC it will allow you to skip any files that have issues and CCC will list the files not copied in its log.

Make sure to disconnect all other external devices. If you are using a hub, then connect the drive directly to the laptop. If the drive is connected directly to the laptop, then try using a good brand of powered USB3 hub which can provide extra power to the drive and the hub also acts as a filter & buffer.

To attempt to check the health of the external hard drive run DriveDx. You will need to install a special USB driver in order for DriveDx to access the external drive's health information. Even with this special USB driver not all external drive enclosures allow for accessing the drive's health information. Post the DriveDx report for the external drive here using the "Additional Text" icon which looks like a piece of paper.

If you can manually copy files from the drive, then ddrescue is definitely capable of cloning the drive. I've successfully cloned & recovered data from hard drives that had such bad failures that they couldn't even be accessed by the OS and were making dreadful noises.

Plus if you are able to manually copy files using the Finder, then Carbon Copy Cloner is capable of copying the files more efficiently.

Keep in mind the more that you use the drive the worse the drive failure will likely become so be extremely careful.

Edit: Also ddrescue is meant to clone the whole entire drive. After the clone is completed, then you will need to attempt to access the data from the good drive. This process can take hours or even days to complete the clone of the failing drive depending on the size of the drive and the extent of the failure.

If you have access to a Linux machine, including a raspberry pi, you could try ddrescue

https://en.m.wikipedia.org/wiki/Ddrescue

You may be able to install ddrescue on your Mac, but I never tried to.