I found this thread while searching for an answer to my specific issue with a microsd card on my Samsung Chromebook Plus.
I am trying to transfer some downloaded videos [they are in Downloads on CB+] to my microsd card. I keep getting the following message: "Copy operation failed. The file could not be modified." I read that it might be a problem with the size of the videos, (about 75mb each) but even a simple Word or PDF document has the same issue.
Just in case it helps ... The sdcard originally was on my Microsoft Surface 3 and contains files saved originally on the Surface 3. I just ejected it there and loaded it on the CB+. I can view files (movies, etc) on the CB+, but cannot copy anything to it.
Thanks and blessings for your help.