"macOS can't repair the disk" is one of the messages you will receive when Disk Utility First Aid failed. Sometimes, it would be followed up with this error message: "You can still open or copy files on the disk, but you can't save changes to files on the disk. Back up the disk and reformat it as soon as you can".
In such a situation, there is a high possibility your disk is damaged. You can run Disk Utility First Aid again to fix your disk. If Disk Utility First Aid failed again, your disk may be corrupted severely. To make it usable again, you can reformat it. Since reformatting will erase all data on it, copy files from it ASAP.
Check if your files that are stored on the Seagate Expansion drive can still show in Finder. If you can open and copy them just like the error itself states, you can directly clone the drive with iBoysoft DiskGeeker in an easy and efficient way. If the Seagate drive can't be accessed and your files on it can't be opened normally, then you should use data recovery software to retrieve files.
iBoysoft Data Recovery for Mac is professional data recovery software for Mac, which can recover data from corrupted external hard drives, SD cards, USB flash drives, memory cards, memory sticks, etc. What's more, this program can recover lost data from unmountable macOS partitions, encrypted APFS boot volume, etc on macOS 13/12/11/10.15/10.14/10.13/10.12 and OS X 10.11.
Now, do the following to get files back from your disk that Disk Utility can't repair:
First, free download iBoysoft Data Recovery for Mac and launch it on your MacBook.
Next, click on Storage Device from the sidebar. From the storage device list, choose your Seagate drive.
Finally, click Search for Lost Data to scan all lost files on it. If you're lucky enough, the lost files will be listed in this app, you can select all the files you need and click the 'Recover' button to recover data and store them on your MacBook Pro or another healthy external drive to avoid data overwriting.
If your data is safe now, you can format the external drive on Mac to use it again.
