Hello!
I had the same problem on my Toshiba 1TB Canvio Basics USB 3.0 Portable Hard Drive. I searched and tried all methods that were available online and none of them worked. This issue came up after I tried copying over 100 GB of data to my portable hard drive from my PC.
My Solution:
Uninstall and re-install the driver. To do this, do the following:
1) Go to My Computer.
2) Right-click on the hard disk drive.
3) Go to the Hardware tab.
4) Make sure the hard drive is selected and then click Properties at the bottom.
5) Go to the Driver tab.
6) Click Uninstall button.
7) Click OK.
8) Follow prompts to Restart your computer. Note: Leave the hard drive plugged into your computer when you restart.
9) After the computer reboots and you log back in, Windows will now install the driver.
10) Once it has finished installing, you should be able to write onto the hard drive again.
These steps worked for me, and I hope they work for you too. I understand how frustrating it gets, especially if formatting your hard drive is NOT AN OPTION!
All the best!