no more space on device while writing to exFAT folder on MicroSD card

JasonR:/ $ su JasonR:/ # cd /stor*/emu*/0/WhatsApp JasonR:/storage/emulated/0/WhatsApp # cd Media JasonR:/storage/emulated/0/WhatsApp/Media # ls -al *.jpg -rwxrwx--x 1 root everybody 506827 2019-02-22 11:42 DSC_20190102_130645.jpg *## I cant' move a regular file in whatsapp Images folder* cp DSC*.jpg "WhatsApp Images" cp: cannot create regular file 'WhatsApp Images/DSC_20190102_130645.jpg': No space left on device *## Neither can I create a 0 byte file.* JasonR:/storage/emulated/0/WhatsApp/Media # touch 1.jpeg JasonR:/storage/emulated/0/WhatsApp/Media # cp 1.jpeg "WhatsApp Images" cp: cannot create regular file 'WhatsApp Images/1.jpeg': No space left on device *## du command o/p* du -sch "WhatsApp Images" 2.0G WhatsApp Images 2.0G total *## Df command o/p. The BindFs between internal storage and SD Card can be seen.More details in Mount o/p below* JasonR:/storage/emulated/0/WhatsApp/Media # df -h "WhatsApp Images" Filesystem Size Used Avail Use% Mounted on /mnt/media_rw/76B2-B58E/WhatsApp 109G 65G 45G 60% *## I m listing top 5 elements in that directory* JasonR:/storage/emulated/0/WhatsApp/Media # cd "WhatsApp Images" ls -al | head -5 so just to give you an idea of my situation.
as is well known WhatsApp will not let you specify where to store its huge volume of media and database files i.e. WhatsApp folder in user internal storage. so I have to use a bind approach to link to a folder on my exFAT MicroSD card with the same name.
all was well for almost 2 years and all of a sudden out of Nowhere I had the situation where WhatsApp images i.e. the "WhatsApp Images" folder ( WhatsApp/Media/"WhatsApp Images") will simply refuse to accept any more jpg from WhatsApp if I un-bind this relationship and have everything downloaded on my internal memory the jpg's & other media are downloaded just fine. but internal storage is crammed - I don't have space.
binding does not cause this to happen to other media i.e. video documents etc. this only happens to images folder and presumably that is related to large number of image files already in that folder.
exFAT does not have such a low ceiling for number of files you can put in a folder.
also just to complete the picture, I can write to any other folder on that micro SD card through binding on internal storage what I did * renamed this folder ( folder.old )and created a new folder and moved all files into that new folder also called 'WhatsApp images' and after about a week the same issue crept up on this new folder * since most of the space is hogged by videos , I created a bind only for video folder ( WhatsApp videos) everything else could go to internal memory.
the result was videos could not be downloaded. so I could bind only the parent WhatsApp folder lock stock subfolder approach fails so my questions are: * exFAT does not have restrictions for such a small number of files in a folder what is causing this issue. on command line it shows up as no more space available
I have attached screenshots of my terminal emulator
* why does in subfolder approach not work. I'd assume that has to do with temp files created elsewhere while you download the video and then they have to be joined some place on the same partition and so the binding does not work . if at all this is in fact true then where does WhatsApp store temporary files before it joins them so this temp folder can also be linked through a bind relationship