Clearing or resetting the Backup memory will erase any User Songs, Registrations, and User Styles from the system backup memory, but it won't erase anything from the internal flash memory.
Clearing or resetting the Flash memory will erase any files that were transferred to the internal flash memory using the Yamaha Musicsoft Downloader, such as MIDI song files and style files, but it won't erase anything from the system backup memory.
A full factory reset would require doing both types of resets, but if you don't need to do a full factory reset then you can reset either just the backup memory or just the flash memory.
Either way, before resetting you should save anything you don't want to lose.
Before resetting the backup memory, you can either save the memory to a User File on a USB drive, or save it to a Backup File on a computer using the Yamaha Musicsoft Downloader.
Before resetting the flash memory, you can save any files that were transferred to it by transferring them back to a computer using the Yamaha Musicsoft Downloader.
The irony is, if you need to reset the backup memory to fix an issue caused by corrupt data within the memory, you will most likely just revert back to the corrupt data if you restore the backup memory after resetting it.
On the other hand, if you reset the backup memory and it doesn't resolve the issue, then you know the problem was not in the backup memory's contents, so you can safely restore the backup memory.
Now, having said all that... I'm not sure where you're getting the "No data" display, but one thing to be aware of is that the keyboard will say "No data" if you try to select one of the User Style slots that hasn't had a style file loaded to it yet. Basically, it means there's no style data in that particular User Style slot-- although that isn't entirely true, because Yamaha actually puts a "minimal" style there that is empty except for having a drum beat on each beat of the measure, sort of like a metronome. So if you're seeing "No data" when trying to select a User Style, it's not actually a problem, just an "empty" User Style slot.
But if you're seeing "No data" when trying to read the USB drive, that's something else entirely.
By the way, a couple of years ago I created a "No data .sty" file by saving a User File that had some empty User Style slots in it, grabbing the bytes that contained the "empty" style data which Yamaha uses for those slots, and saving them to a style file. I did that so people who wanted to "clear" an individual User Style slot could load the "No data .sty" file into that slot. So if you happen to have that style file on your USB drive, you'll see a "No data" style when browsing the USB drive for a style file to load-- although you should also still see any other style files that are on the USB drive.
Not trying to confuse you, just checking whether you do actually have a problem or not.