I was able to fix the error -43 on my Sony
I received the error -43 on my PRS-300 last night. I figured out how to fix it with my Macbook. I noticed the problem happened after I pulled the usb connection without waiting for a proper eject.
To make a long story short, the fix was to plug it back in and bring up the disk utilities. I chose the Sony disk (flash drive) and clicked "Repair Disk". After that the error -43 issue went away.
I believe OSX leaves some garbage behind when you do not eject properly. I tried to remove those hidden files by hand from a terminal window but some of them couldn't be deleted. The repair disk utility saved the day.