You may also get ‘linux_raid_member‘ instead of ‘isw_raid_member‘, usually it’s the same.
So you probably have run through other tutorials telling you not to mount directly but to mount the raid instead…. but you have no raid!
Maybe though, you HAD a raid back in time, and you removed it leaving the disks as separate storage, and in that case there would be “rogue” raid metadata on the disk that makes linux (mint in my case) think it is a raid, and get confused.
Thanks to Chris_F I finally solved it, you need to use dmraid (install it if it’s not on the system, via apt-get) to remove those orphan tags:
sudo dmraid -rE /dev/sdb
In my case, this instantly showed the disk in file manager.