My apologies, dwanthny. I thought I mentioned this in my first post but after reading it over, I found -- lo and behold -- that I had forgotten to include it! Red face
Kovid is right. I read the "fine print" on the Overdrive site last night and discovered that they have specific instructions for Sony Readers:
Overdrive REQUIRES that Sony's own Reader software be used to transfer the ebook to your Sony device. If you use any other software to send the ebook to your Sony, the Sony won't open it. (Maybe Overdrive has a legal agreement with Sony?)
Overdrive does not dictate specific ebook software for most other ebook readers (Nook, Kobo, etc.), only for the Sony. (I assume these devices don't have their own proprietary ebook management software.)
So this is not a flaw in Calibre. It's a restriction imposed by Overdrive.
As Kovid says, the only way around it is to
- accept Overdrive's restriction and use Sony's Reader software to send the ebook to your Sony, or
- use a DRM-removal tool on the Overdrive ebook, in which case you can send the book to your Sony Reader using Calibre (or any ebook management software you want), and it will be readable on the Sony.
A 3rd option (might not be available to everyone, depending on your local library) is to borrow from a lending site that doesn't have Overdrive's restriction (such as EBSCOhost).
___________
P.S. Like all ebook readers, the Sony Reader still requires that you have Adobe Digital Editions installed and that your computer/devices are authorized with it.