[arch-general] boot from install medium - mounting additional partition refused

[Friedrich Strohmaier]

Hi archers,

today i was booting from the install medium dd'ed on an usb drive.

As there is space left on the medium I created one more partition to load or
save stuff from the system booted. Surprisingly I cannot boot:

root at archiso ~ # mount /dev/sdb3 /mnt
mount: /mnt: /dev/sdb3 already mounted or mount point busy.

I could not find an obvious reason for that:
/dev/sdb is the usb device.

root at archiso ~ # lsblk
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
loop0    7:0    0 489.4M  1 loop /run/archiso/sfs/airootfs
sda      8:0    0 186.3G  0 disk
├─sda1   8:1    0     1G  0 part
└─sda2   8:2    0    10G  0 part
sdb      8:16   1   1.9G  0 disk /run/archiso/bootmnt
├─sdb1   8:17   1   1.1G  0 part
├─sdb2   8:18   1    64M  0 part
└─sdb3   8:19   1   792M  0 part

root at archiso ~ # findmnt /dev/sdb3
1 root at archiso ~ #

root at archiso ~ # findmnt /mnt
1 root at archiso ~ #

Does anybody know how to nail down this one?
-- 
Friedrich