how to unmount disk after guix system init error?

[znavko]

Hello! I am trying to install Guix on other notebook. when I've parted it with 
fdisk using such partitions:
/dev/sdb1 50Gb Linux filesystem
/dev/sdb2 9Gb Linux swap

then `guix system init /mnt/etc/config.scm /mnt` gave the error:
Embedding is not possible. Grub can only be installed in this setup by using 
blocklists. However blocklists are UnreLIABLE and their use is discouraged.
guix system: error: failed to install bootloader

And this makes me sense need to do another partitioning like this:
/dev/sdb1 150Mb BIOS Boot
/dev/sdb2 50Gb Linux filesystem
/dev/sdb3 9Gb Linux swap

I cannot do this. I fullfilled this but making filesystem on /dev/sdb2 gives 
the error, that device is mounted.
As I understand this is because of my run 'herd start cow-store /mnt' and also 
run `guix system init .... /mnt`
I've stopped herd service. But do not know what to do else. So I can only 
reboot and start again.

Please is there another way to solve?