Hi I have almost the same problem
I have an HTC One V (no SD card, but plenty of internal memory available). I can take pictures, but when I take a screenshot or whenever I try to send anything via whatsapp or download any picture/video/audio file that my whatsapp contacts send me it says:
"Cannot download because no SD card is available. Please insert your SD card or unmount it as a disk drive, and try again"
I guess this is due to the fact that Whatsapp stores all the data in the memory card, as default setting. Is there any option to modify this??
Thanks