mint partitioning error

[Dave Stevens]

I have a computer with a system disk running mint 20. I also have a
32gb flash drive with mint 20 installed. Both boot and run ok. The
flash drive has one big 32GB partition. I want to run gparted from the
system disk to add a data partition to the flash drive. I run gparted
as admin (sudo) when the flash drive is not mounted. I have used
gparted many times satisfactorily. I'd like to have the existing
partition on the flash drive reduced to about 10GB after which I'll
allocate the remaining space to a data partition suitably formatted.

I can't get past the part of gparted that should shrink the existing
partition. The options I want are greyed out. There isn't a little lock
icon beside the partition in the partition list. I'm stuck.

The flash drive seems ok otherwise, no errors showing up.
Anyone able to recommend a tutorial or workaround?

Dave