Glo Glo and Calibre

I'm trying to put .rtf files on an SD card to read on my Glo, and am running into a problem. Sometimes the files transfer OK. Other times Calibre says it can't connect to the Glo because of a communication error. This can happen in the same session: one book will transfer OK, and the next one won't.

The files are magazine articles that I edited in Word and saved as rft.

Calibre is ver 0.9.28 [64 bit]
Glo is ver 2.5.1
Windows 8
Lots of room on the SD card

Any help would be appreciated.

Here are the details of the error message:

(1117, 'GetDiskFreeSpace', 'The request could not be performed because of an I/O device error.')

Traceback (most recent call last):
File "site-packages\calibre\gui2\device.py", line 85, in run
File "site-packages\calibre\gui2\device.py", line 551, in _upload_books
File "site-packages\calibre\devices\kobo\driver.py", line 1758, in upload_books
File "site-packages\calibre\devices\usbms\driver.py", line 256, in upload_books
File "site-packages\calibre\devices\usbms\device.py", line 986, in _sanity_check
File "site-packages\calibre\devices\usbms\device.py", line 177, in free_space
File "site-packages\calibre\devices\usbms\device.py", line 134, in _windows_space
error: (1117, 'GetDiskFreeSpace', 'The request could not be performed because of an I/O device error.')