I have been having the same problems since last summer I find that if I put the usb in before I switch the printer to usb from the main menu it tends to see it and then again not always. I also found that if I wiggle the USB up and down or side ways gently it will start to see it then it is just holding my mouth right and it loads. It has been an ongoing problem to the point that I finally (after 3 years) figured out my password to the wifi network and connected to the printer from my computer. I am still trying to figure out how it works but have successfully added and removed files.
One problem I had last week was when I did this on the wifi I had used a file and opened it, then I added a few more watch band link items that I had on the print I just copied on the original pieces there after a customer said they need more printed than before plus I increased the scale after noticing I hadn't before then resliced it using the same file and overwriting it. I transferred it to the printer chose the print and I could see that it had more links on the print so I hit go and come back 15 min. later only to see it printed the old one without the added pieces did this 2 times before I finally resliced it again using a different name deleted the old one and it finally printed all of the items.
back to the USB just jiggle it gently one side to the other or up and down very slightly, a few times I have had to hold it in place with slight pressure up to get it to load then transfer it to the library. For some reason ( now it will jinx me) it has been working fine but seems slow to load. Could be that it is because I have so many files store on my USB.
I suggest that the software itself on the computer should have a storage file associated with it. Instead of deleting files or prints it should give you an option to archive them to later retrieve and transfer to the printer rather than just deleting them from the puter itself.
While I am here on this subject I have set up how the computer reads the USB files by date rather then by name, but the printer doesn't look at it that way when it sees the USB so you have to scroll down through all the names to get to the file rather than it being at the top of the list by date.
But when you transfer it to the library it is at the top of the list of prints to be loaded.
I would like to see a change in the software to see the USB by Date. I am not using the new win10 B9 software for my 530 so this would need to be an update for the other Windows versions before win10.
I'm so far behind I think I'm First !!