RAID1 default disk capacity is lesser than the disk sizes. Any ideas? Intel RRAID Storage Technology - RAIDfrom Bios

The default size of 884.9GB is exactly 95% of the smallest disk, which is 931.5GB. You can manually change this value to the full 931.5GB if you wish to do so.

This feature is documented at https://www.intel.com/content/dam/support/us/en/documents/ssdc/ssd-software/RSTe_NVMeProduct%20Spec.pdf. Its purpose is to protect against NVMe of different sizes. Your current disks are 931.5GB but the next one you buy (if it is a different vendor) could be 931.4GB. A smaller disk cannot be used to replace a bigger one in a RAID1 array. Rounding down the size to 95% gives you a bit of leeway there, but you are free to make it use 100% the space.

To quote the relevant section 2.6.3 of the document above:

Disk Coercion

The Intel RSTe NVMe will provide support for Disk Coercion. When a RAID volume is created, this feature will analyze the physical disks and will automatically adjust (round down) the capacity of the disk(s) to 95% of the smallest physical disk. This allows for the variances in the physical disk capacities from different vendors.

A pure speculation, based on personal experience with similar software (I avoid using RST for a lot of reasons):

931.5 * 1000 * 1000 / 1024 / 1024 = 888.35 + some rounding errors

Looks like the old 1000 vs 1024 dualism in hard disk volume labels.

The usual IT thinks that 1k = 1024 and 1M = 1048576 (1024 * 1024). 1024 is a good binary number, it looks like 1000000000 in binary and is handy for IT calculations.

Disk manufacturers prefer 1k = 1000 and 1M = 1000000 (exactly like the case is for SI units). This gives bigger numbers on the label and bigger numbers sell.

When one wants to be sure to imply 1024 multipliers, Ki, Mi and Gi abbreviations shoild be used (usually pronounced kibi-, mibi- and gibi-).

https://en.wikipedia.org/wiki/Byte#Multiple-byte_units

In your particular case:

The disks are advertized as 931GB.

The "SELECT DISKS" menu shows the size in manufacturer units for the sake of correspondence between the label and the number on the screen.

The "CREATE VOLUME" menu shows "IT units", because... whatever the designer of this software package imagined.

The real overhead of the RAID 1 volume (spare for the half used for redundancy) is like 512 or 1024 (or probably even 4096 for the sake of the advanced format) bytes and is completely negligible (and the numbers above are not accurate enough to show a difference that small anyway).