You have a misunderstanding here. And to stress what @Tanningtom already explained:
When it is mounted, it is in use and you should not try to create a new file system (for former MS-DOS users: format) on a mounted file system. Very logical, because that action will first and for all destroy the file system you have. And then create a new one, that then (hopefully) can be mounted.
Thus, when you want to create a new file system on the partition on the device, on the Tablet (whatever operating system might be there):
do NOT mount it there (well, when it is mounted, the tool you use to create the file system might unmount it itself without reporting such a basic action to you);
take not what the file system type is; you should be aware of that, specialy when you want to use it for exchanging with other operating systems.
Creating a new one will aotomatically destroy the old oen (there can only exist one at a time on the same partition ).
What I would do is connecting it to my openSUSE system and start YaST > System > Partitioning. But it depends on what type of file system you want to create. I am not sure that YaST can create many types (if at all) of non-Linux file systems.
Thank you. I used Partitioner to delete the troublesome partition, created a new partition using all available space and formatted it with FAT. Then I copied a simple file to it which was readable on the Tablet. It mounts correctly on the computer and files are visible in Dolphin. There are no error messages in Dolphin.
One problem remains: all the files on the microSD are owned by root even though they were originally owned by a user. They cannot be opened by the user on the Linux computer. What is the easiest way to make all files on the microSD available to a normal user?
I did read the link you provided earlier. I am familiar with most of the information but do not use it regularly enough to know exactly how to use it in practice.
This is a FAT file system and thus has no knowledge whatsoever about owners and permissions. That root seems to be the owner is faked by the mount.
In most cases when you connect such a device, it is done by a user using a desktop. In that case, the mounting is done “through the desktop”. And that mount is done on behalf of the user running that desktop. And thus that user will be the owner of the files. (BTW, IIRC this story is also in the link I gave you :().
For some reason you did different and mounted while being root. That is not a problem, because there are parameters root can use to mount it with a user as owner.
specially the section
Mount options for fat
You will find there e.g.
uid=value and gid=value
Set the owner and group of all files. (Default: the uid and gid of the current process.)