Looks like no one is going to answer this. My thoughts are: try a different usb stick to see if the issue corrects. And what if, like SSDs, you can't write to a memory location until it is reset to zero. With SSDs we have to enable Trim, which is software and goes around reseting deleted memory back to zero. Bottom line. I hate USB sticks. They're slow and unreliable. I've installed front Sata docks in all my computers. If I like, I can copy 1TB of data to a notebook (pocket) sized Sata drive, which is seen as an internal drive. I'm getting 90MB/s copying from one Sata drive to another. I can use grsync to make and update my backups.
Since SSDs need Trim, I use a SSD for my system and programs. My files are stored to regular spinning drives. So the SSDs are essentially read only and the magnetic drives are read write.