Thank you TriX, you can imagine how I felt after shooting 2300+ pictures but only 600+ show up.
Suddenly, I remember I am an uglyhedgehogger and looked further into the 64G which I thought is faulty.
Now I am a happy hogger, especially for the fast response.
But can you advice me how to view hidden files?

Answer:
1) I don't know how to view hidden files in Explorer
2) I have been using Faststone to organize my photo; FS can show Nikon's raw file NEF
3) Since I reformatted my 32G(jpg); I used it to shoot a few more hundreds
a) I haven't used the copy(64G) to run a chkdsk.
b) I haven't used Recuva on my 32G

Meanwhile, I did something hinted by Bill de,
I have my 64G in my camera and did Menu:Playback:hidden images: deselect all; suddenly I can see my pictures of day 1.
So I viewed it on Explorer, but again, no folders show as all are hidden.
Then I reinserted the 64G in my camera, and did once more Menu:Playback:hidden images: deselect all; I can see my day 1 pictures again.
Then I used a cord connecting my camera to my PC: now I could see the missing folders
I copy and paste of the lost folders from the camera to my PC
Now it seems that I can see all the files on my PC.
Thank you folks, as your hints did help me a lot to do this recovery, as I was quite saddened earlier and almost give up hope.
It is an horrible and scary experience to place all my eggs in one basket.
I don't know why this has happened, and please explain how to view those hidden folers/files

yes, the copy indicates exact number of files and only one folder is shown(56+G and 2000+ files)
I insert the disk back to my camera, and still only the photos of original one folder are shown

Hi Trix, I made a copy of my disk and did what you recommend.
Now, the remaining folder disappears. I assume all 4 folders are hidden.
I believe the data is still there as the property of the SD still shows 56G of data.
Please advice me how to recover those hidden files.

I don't think there is anything.

I have been on vacation for a month and had been shooting with my D610 with a dual SD disks of which one is 64G for raw and another 32G for jpeg. During the course of my trip the 32G was pronounced full so I reformatted that disk thinking I have everything in my 64G and continued shooting.
After I came back from my trip I found out through Window Explorer and Faststone that some folders in my 64G are missing and I have lost more than 10 days of photos.
In my 64G, the DCIM shows 56.6G of 2,335 files in 4 folders, but inside DCIM, only one (249ND610) shows 14.4G of 606 files. Nothing in the Lost.dir. Where are my other 3 folders?
I am so afraid that that 40G+ of photo data has been lost permanently; stupidly I have reformatted the Jpeg disk, hence I don't have any backup.
Does anyone know how to recover those lost photos or any hints? Much appreciated in advance.

leftj: Do you make money on your photography?
You have to understand many of us are amateurs, we have to live with less than perfect gears.
I am an aged retiree, so it doesn't matter if the color is off a little bit, the speed is a little slower or sometimes jammed.
I spent less than $1000 4 years ago for Dell desktop and a 40" Samsung monitor to do my photo editing.
And I won't blame my gears for not producing good photos.

Beware of Apple repair cost, or extend memory cost, my son spilled some wine on his Mac keyboard and blown the power. The Apple store wanted $900 to fix it.
So my son opened up the keyboard and used light to dry it up for several days. Since it was not considered water damaged then and the Authorized dealer fixed it under warranty for free. It was a blown fuse problem after all.
Apple makers make those other MS products too.
https://www.npinc.ca/who-makes-dell-hp-toshiba-acer-apple-laptops/

My 2cents advice: if you might use it on hiking trip, consider a Zomei with ball head or brands with similar construction.
One leg can be used as a monopod, and 17" folded into your backpack.
You can have a choice of turn or snap lock, carbon or alum.
Very solid and won't break your bank.
You don't need to spend on one over $500 which you can hang yourself.
Just follow good utilization of tripod, firm ground, hang your camera bag at the center hook, and use delay shutter, I think it is good for most situations except in the middle of a storm.

Besides all the good ideas, try this also: use your smart phone in pano-mode, horizontal position and pan upwards; you don't have to finish the whole pan-range.

search Ebay: Camera rain cover and you will find one that you like. I keep one in my bag but never use

I wonder if you checked the outcome of each picture taken?
If there were no card, no image would show. I think some camera let you shoot a few and use its internal memory so as to test out the camera without a card.
I hope you still have your memory.
You may download some from the web and keep them as your own.

If not the smartphone manufacturers were so aggressive in improving their photo technology...one day they will surpass dslr with adoptive lens and bigger sensors.
Remember the Kodak demise.
Big Dslr manufacturers beware, your loyal buyers are dying, literally speaking.

It would be a good selling point if the manufacturers allow their users to customize and download their up-to-date firmware.
There are enough competing manufacturers out there trying to gain favors of the majority both simplicity and complexity. Then you will hear someone says" I choose to switch to this brand because they are always trying to update the firmware and allow free downloads; so my gear will not be left as orphans".
Being generous to its users will increase their popularity.
Remember how big Kodak once was?
Then why not?

I understand many prefer simplicity and many complexity.
If one shoot only manual mode and focus, can he buy such functions only camera?
If a manufacture custom make such camera, that would cost a lot more and no better than those with other add-on modes/functions.
The point I want to make why not the manufactures throw in as many functions(which only requires firmware cost) to satisfy a more variety of customers.
A dslr with similar functions and features of a modern P&S should produce better results respectively. Then why not?