Incomplete file transfer when copying from laptop to external SSD

Zorin Pro user here:
When I copy files from my laptop to external SSD it'll say files succesfully copied. I'll look on the SSD and compare sizes, everything's fine. I'll even play the freshly copied media on the SSD, no problems. Then when I unplug the SSD and reconnect it, the file is 10% of the size it should be and some files haven't transferred at all. I thought it might be a file manager issue so I tried Thunar and problem persists. Any ideas? SSD appears to be fine because it handles transfers from windows no problem. For some reason transfers to a spinning/mechanical drive are fine...

Is there an app or methodology to ensure file integrity?

You can run a S.M.A.R.T. test on it. What File System is the external SSD formatted to?

You could try copying via the command line and then type “sync ” in a shell command line to make sure all buffers are flushed to the drive before ejecting the drive. This sometimes takes a moment or two.