Lenovo laptop: Linux Mint 17.3 Mate, USB flash drives are write-protected or require root privileges.
Today, I solved the issues with usb flash drives. Everything suggested in the Mint Forum posts was tried, up to March 2016 (no success).
Some background. The issue has persisted for 5 months. The usb drive is plugged into one of the usb 2.0 ports on the laptop, contents appearing in the file manager, but I cannot copy a file from the usb drive, nor can I add a file to the usb drive. Curiously, I can delete a file from the usb drive. Certain usb devices are excluded from the problem, in particular, an SD card device. Command line file operations from a terminal succeed. Using the file manager Caja in admin mode succeeds. The usb device mounts with rw flags, as documented in a terminal with the "mount" command. On a desktop with the same OS 17.3, the issue did not occur.
The issue had to do with options for a file manager, done some time ago. In a compromise, I selected a file manager (see how below). It worked. The GUI launched the file manager and everything seemed normal.
But not for some usb flash disks. Surprisingly, usb devices for SD cards worked.
In Linux Mint's File Manager setup, Mate GUI, buried many levels deep,
System ==> Control Center ==> Preferred Applications ==> System ==> File Manager,
there were 3 options, like an extra door at the gates of Hell. Two of them were labeled "CAJA," Linux Mint's file manager, a poorly re-constructed Nemo (==Nautilus). The third label was "FILES."
Today, I chose one labeled "CAJA" and tried it. It solved the usb problem. Was it Caja with admin privileges?
The correct door out of the gates of Hell.