I have spent some time researching this. I believe I know where the problem may lie. I bought a Samsung 64GB micro SD card as described and installed it in my SGS4. Although I have not experienced the behaviour as described exactly, I have had a number of problems with the card in that it would often corrupt video and photos recorded to it. I have done some research and it appears that the slower (and big cards) are just not fast enough to keep up with the phone(samsunggeeks.com/2012/05/20/micr ... rLjP9jxvGg) I have since purchased a 32 GB super fast (class 10) micro SD card and it has been running fine.
So, I would recommend ditching the 64GB and get a class 10 card and report back on how it goes.
When I installed my SanDisk 64 GB micro sdcard in my Samsung Galaxy S3, it would not see the sdcard. I tried using the windows format utility with no success. I think windows has some limitation formatting SD cards over 32 GBs to fat32. I also tried using several other formatting utilities on the web that didn't seem to work either. I fixed my SanDisk 64 GB micro sdcard by reformatting it with Gparted linux live iso. I think Gparted is the best utility overall to fix any corrupt sdcard, so give this a try.
If you have an Intel processor, go into the bios and enable virtualization technology/VT-X. It's disabled by default.
1.Load the Gparted iso in Oracle VM VirtualBox. You'll need to hit enter a few times to load into gparted.
2.I connected the micro sdcard to a Kingston usb 3.0 sdcard reader. (I think any USB card reader will work.)
3.Loaded the USB device in virtual box.
4.Format again to 64 GB FAT32.
Hope it helps.
I have about 13GB free of 64GB with backups,photos, music, tv shows. I've been using the 64 GB sdcard for about 8 months now without any issues. I get about 12MB/s using MTP and about 4.5MB/s using FTP on my Galaxy S3.
Well I don't know if this adds fuel or is off topic......but I get the same problem, Dopus hanging viewing mtp device, but not from file copies. For me it always starts after I have deleted a file or two. Then Dopus just stops responding, the file list doesn't change (doesn't remove the file I deleted) and stops responding tofurther deletes or changing folder. F5 to refresh it comes up Reading Folder. What happens next depends on device:
Galaxy Tab S2 (Android 5.0.2): it stays hung for exactly 1 minute, then it updates and responds again. Once responding, I can copy files, delete as many as I want, the problem doesn't show up again. If I am impatient I unplug/replug the USB and it responds immediately again.
Galaxy S5 Mini (Android 4.4.2): it stays hung forever. I unplug/replug the USB and it responds immediately again.
It happens working on external card or internal storage. Neither is encrypted.
If I turn MTP off ie. revert to Explorer handling, this does not happen.