Hello tshh,   I think you have it with your last question.   If you have only replaced some drives in the RAID6 array with larger drives, you would only be able to use up to the capacity of the smallest drive in the array.  If you change out all the drives to larger size you should then be able to do the Online Capacity Expansion.  
I have an R720XP with PERC H710P mini controller). This has a 24 disk array configured as RAID6.  All the disks were 600Gb SAS disks. I want to increase the total capacity of the virtual disk, so I have replaced some of the drives with 1.2Gb SAS disks.  The array has successfully rebuilt and is healthy, but the extra capacity is not showing. In iDRAC and OMSA it shows the correct total capacity, and the 'used' capacity is correct based on the disk size it replaced, but the 'available' space is showing as zero. [IMAGE] I've tried using 'Reconfigure' option in VD setting, but this only gives RAID0 and RAID5 options, it doesn't allow me to keep as RAID6 but increase the volume size. Any tips on what I'm doing wrong? Do all disks in a RAID6 array have to be the same size? Thanks!    
Increasing RAID6 size (PERC H710P mini)