Thanks for your swift reply.
First, the information about the broken disk disappeared once I reseated the physical disk, so the problem that is remaining is that of an unassigned disk.
When running storage disk show -shelf 11, disk 2.11.4 is missing :
2.11.3 836.9GB 11 3 SAS aggregate BL_NF0011_DATA1 BL-NF0011
2.11.5 836.9GB 11 5 SAS aggregate BL_NF0011_DATA1 BL-NF0011
Any command with disk id 2.11.4 fails because it can't be found.
I found out by checking the serial number that the disk in Shelf 11, bay 4 is an unassigned disk with the name :
BL-NF0011:switch_V2_2:6.126L83
but assigning it to node BL-NF0011 fails because "redundancy errors were detected" or maybe it would work if I use the force option ? The other thing I haven't come across earlier is the disk id. In the past when I replaced a broken disk, the new one alsways came up as eg. 2.11.4 and when I execute "storage disk show BL-NF0011:switch_V2_2:6.126L83" the Stack ID/Shelf/Bay: - / - / - information stays empty so I'm puzzled how I can find the physical location of a disk with such a name.