I have a few external USB drives I use for running backups with my MacBook (in addition to Time Machine). Since upgrading to Sierra, my permissions have changed: I can no longer write to them, just read them. They are NOT NTFS-formatted, so that's not the issue (and again, I'd been writing to them for more than a year, no problem). I can read and write fine from my PC, so it's not a disc problem, it's clearly something with how the OS is changing permissions. How can I change permissions back so I can use these backup drives? Funnily enough--and fortunately, considering how often Macs seem to die--my Time Machine drive is still working great. It's a 13" Macbook Pro running, yes, Sierra.