I'm getting a similar error as well - PRS-505. Here is my exact message:
Error communicating with device
The reader has no storage card in this slot.
Traceback (most recent call last):
File "site-packages\calibre\gui2\device.py", line 56, in run
File "site-packages\calibre\gui2\device.py", line 233, in _upload_books
File "site-packages\calibre\devices\prs505\driver.py", line 125, in upload_books
File "site-packages\calibre\devices\usbms\device.py", line 734, in _sanity_check
ValueError: The reader has no storage card in this slot.
Using Calibre 0.6.37. According to Calibre I have 177mb free. I can successfully move files onto my reader with Windows Explorer and using Reader Library.
I'm not getting an error after ejecting the reader.