Probably the easiest thing to do is to de-authorise your Reader, and then re-authorise it ensuring that you use the same Adobe ID (e-mail address) and password as you are using on the computer that you transfer your eBooks from.
Connect your PRS350 to your computer - the Reader Library software should start on the computer. Then click on 'Reader' in the menu down the left hand side of the screen (so that the main window shows the contents that you currently have on the Reader). Then press the following three keys simultaneously on your computer's keyboard: Ctrl+Shift+D. This should de-authorise the Reader (you can check to see if this action has been performed successfully by clicking on 'Status' in the left hand menu on the computer). One final check - if you disconnect the Reader from the PC, go to the home menu and select 'Settings' and then 'About'. Under Device Information you should see that it says that the device is NOT authorised for Adobe DRM content.
Your Reader can now be authorised again - but making sure that you use the same ID as the computer. To do this you can drag and drop a copyright protected eBook (one that you've purchased) from your computer to the Reader - you should see the prompt to authorise the Reader appear automatically.
I had a very similar problem. My laptop died, and when I replaced it, I couldn't transfer any new books to my ereader as it said the ereader was authorised for another user. I couldn't remember my original login, so couldn't follow the procedure given by SOny, and it looked as if the ereader was now beyond use.
I followed the procedure described by Druzman, and it now appears to be all working again. So many thanks
Thank you, you are amazing! I have a PRS505 and have had the same proble , being unable to transfer my new books to my reader when using my new laptop, as the reader was allegedly authorised to another user.
After 2 days searching for any clue as to what to do ...( and a few choice words and an increasingly short temper)... I came across this discussion by chance and solved the problem in less that a minute!
Your instructions are brilliant..... they should pay you to give advice on their behalf as their help sections are extremely unhelpful.