The jpeg shown in the camera's preview is the basic Jpeg embedded in the image file being reviewed.
Both Jpeg and NEF or other RAW files include these preview thumbnail files within the image. This is why your computer can show a thumbnail image in the file manager or show a preview image for a RAW image.
I understand that. In my question I was referring to the JPG that is produced when you have the primary card set to NEF and the backup card set to JPG in the D800. When I delete a NEF that I don't want to save, the JPG saved to the backup card is still on that card, and it shows up in the reviews. I was looking for a way to delete the two files simultaneously.
I just found some information on this in the manual (see page 85). It looks like what I'm describing (deleting the NEF sent to the primary card doesn't delete the JPG sent to the backup card) is the design. Ergo, there isn't going to be a fix. Oh, well ...
Ok, there is what I found in the manual, page 221:
If two memory cards are inserted, you can select a memory card for playback by pressing the W button when 72 thumbnails are displayed. The dialog shown at right will be displayed; Highlight the desired slot and press 2 to display a list of folders, then highlight a folder and press J to view the pictures in the selected folder. The same method can be used to choose a slot when selecting images for operations in the playback (0 259) or retouch menus (0 341) or when choosing an image as the source for preset white balance (0 159).
Make sure you select the CF card for review, then images you delete will not show up.
Yes, when you delete an image, the JPEG backup will remain on the SD card. This is a good thing, if you ask me. In case you accidentally delete a good NEF, at least you will still have the JPEG backup. Those backups don't take up much space, so if you use two cards with the same size you can do a lot of deleting NEFs on the CF card before the SD card gets full. Once you import your NEFs into your computer, the extra backups on the SD card should not bother you, either. So I think Nikon designed it right.
Unless I'm not doing something right, it appears there's something wrong when Image is previewed while using Primary and Backup File Simultaneously. I've followed the instructions for displaying images on Page 221 and 260 to no avail. The RAW and JPEG images still show in Image previews for me when I'm directing the camera to display only the RAW images from one card.
Has anyone else experienced this?
I've also read a similar issue on this and more firmware wishes at dpreview.com