The first 4 drives (disks 0 to 3) lose an extra 13GB off of that 458GB to the OS of the array. So if you have any raid set that includes 1 or more of those drives, instead of being 458GB, they are really about 445GB. So a 4-disk raid 10 is 890GB. Deduct 751 and you're left with ~139GB.

Thank for your reply!

 

Why the hard drive's capacity was changed to 458G from 500G after it has been insert into the Disk Array (where is the 42GB)? And does the lose capacity have rule according to the RAID type? Why 13GB has been lost, but no other numeric?

If I use 3 hard drive (each capacity is 500G) to crate RAID5, then how many is the actual capacity? 500X2/3GB, right?

 

Thanks again!

I imagine you're familiar with how hard drive manufacturers figure drive sizes? Every single desktop and server drive works this same way when figuring out drive sizes. This is common knowledge for sys admins and computer support people (Seagate has a decent article on it here).

"500 GB" = 500,000,000,000 bytes.

For a computer system:

1 Kbyte = 1024 bytes
1 Mbyte = 1024 Kbytes
1 Gbyte = 1024 Mbytes

So 1 Gbyte = 1024 x 1024 x 1024 = 1,073,741,824 bytes.

Now you take the 500GB that the harddrive manufacturer advertises and divide it by this 1073741824 and you get:

500,000,000,000 / 1,073,741,824 = ~465.7 GByte. EMC uses some space for metadata and spare sectors, so you lose a bit more after the decimal to binary conversion and you're left with 458GB.