First, you cannot "shrink" RAID arrays on hardware RAID controllers, either to use less space or to use fewer disks. Second: If boot mode is set to BIOS, you are stuck - you will need to reinstall Windows with UEFI enabled or change the RAID configuration(s).  What OS are we talking about?
I got a PERC H730 configured as a RAID 6 and a single VD was created that contains 6-2TB physical Disk, that gave us a total of 7.5 TB usable space . Now I don't know if I'm doing something wrong but windows server is only allowing me to create 2TB partitions and since we have the OS installed there and the owners want to keep the recovery partition and we have the OEM partition, that is giving me the option to create only 2 more partitions of 2 TB each, so I have about 3 TB that are Unallocated and I can't use it. So, am I'm doing something wrong on windows? and if not is there a way to shrink the disk used on the main VD configured on the RAID so I can create a second disk and reclaim that unused space. I made it as far as the reconfigure menu in Openmanage, but don't know if I can do it from there. Any help will be appreciated. -Jose
Can I shrink a Virtual Disk using Openmanage