partitioning: creating a partition with sfdisk

I am working in a laboratory that needs to format an SD card with two partitions. I formatted the SD card, discovered it as sde1 and then ran sudo umount /dev/sde1 before running:

sudo sfdisk /dev/sde1 << EOF
> 1,,0xE,*

I got the following error:

Checking that no-one is using this disk right now ... FAILED

This disk is currently in use - repartitioning is probably a bad idea.
Umount all file systems, and swapoff all swap partitions on this disk.
Use the --no-reread flag to suppress this check.

sfdisk: Use the --force flag to overrule all checks.

I have no idea where to go from here. I am new to this too. I had read that I needed to disable automatic mounting in Ubuntu and I used gsettings set automount false That had no effect. I would still get the previous error. Please help.