I was having problems copying data from my phone and tablets (samsung's) where the permissions were giving me problems at times and sometimes I couldn't mount the device - so I'm using a OTG USB drive temporarily.

I'll copy files from the phone, tried moving and copying, from the phone drive or SD card and paste into the USB drive. It showed the files the correct size and I've viewed them (opened them) from the USB drive. I'd then close the "my files" and remove the drive.

When I access the drive in Dolphin, Krusader, Conquerer, very often they show 0bytes in size, but the filename is fine and the date is there.

I'll do the same process 3-5 times and they come up as zero bytes. Then they will just work, doing the same thing!

I've tried 4 different drives and this has happend when copying over USB cable, where the files showed as 0 bytes after copying.

IDK if this is an Ubuntu thing or Android thing and it is extremely frustrating!

Has anyone experienced anything like this or know what might be going on?

I've not tried the two-stage process you describe, but I do transfer files -- specifically, image files -- from my Samsung Galaxy 5 phone to my Xubuntu systems. I do so by using the "share" option in the phone's "Gallery" app to e-mail the image to myself. It transfers at the full resolution of the original image, and I then save it from Thunderbird on the Xubuntu box. This might be a solution for you. I've not tried it with text files or other content in "My Files" on the phone but it might be available there also.

But doing what you do send everything through your email account and you need internet I would guess. That is an option when I have wifi access I guess, but there are too many files for me to do this most times or they are often too large with video files.

I'm just wondering if this is an adroid problem or a linux issue and it's been tough figuring out when and where the issue is happening but I do think I remember it happening a few times with Windows as well (long time ago) so now that I think about this it may be an android issue...

maybe check to see if you are copying links (to your files), rather than the actual files.