Raid 1 default disk capacity is lesser than the disk sizes. Any ideas? Intel Raid Storage Technology - Raid from Bios

Here the screenshots I think they explains everything

The disks are advertised as 1 TB and the real size of the disks are 931.5 GB

I have installed windows server without raid setup for experimentation. Both disks are fully working with no non-useable sectors and all 931 GB is available to use.

Edit I have found this link

https://support.lenovo.com/tr/en/solutions/ht507601-intelr-rapid-storage-technology-enterprises-default-volume-size-is-not-maximum-size-lenovo-thinkserver

I also see 95% array allocation after deleting raid and trying to compose again

This could sound weird, yes,

But some RAID systems could do one of the followings or both:

• change block size
• save some space (tough your case looks pretty much) for themselves metadata

in this case, it is better to have a look at the Intel's Rapid Storage Documentation

You are confusing Gigabytes with Gigibytes. Samsung 850 Evo's are advertised as 1 TB SSDs. Not 960GB. 1TB under the marketing blanket is actually 931.323 Gibibyte.

Marketing departments like to round everything up, as it seems as more storage. Just use a quick gibibyte to gigabyte converter to see that 1000 Gigabytes is actually 931.323 gibibytes.

Your raid controller sees everything in gibibytes not in gigabytes.