sync hangs

[Richard Kimber]

I'm having problems accessing a usb card that I've been trying to write
to.  In Caja I get an error saying "An operation is already pending"
If I type sync on the command line it hangs.
I see from ps that there are three sync processes in uninterruptible
sleep and I can't kill them.  I assume this is the problem.

What's the best way to proceed?  I'm using Ubuntu Mate 18.04 with
4.15.0-24-generic

- Richard.
-- 
Richard Kimber.