I have the same issue since I switched from my 4GB to a 16GB Sandisk Class 4 from Amazon, too.
The issue exists in the Clip+ AND in the Fuze2.
The original firmware has no problems with the card at all.
ROCKBOX CAN read the card if you fiddle around with it long enough (reboot, reinsert). When youre lucky it gets recognized and then it works flawlessly until you shut the unit down.
When you remove/reinsert the card the SD-Card symbol flashs.
Cause the card is working without any problems once its recognized, I think it must be a timing/initialisation issue.