Scottj22,  When it comes to the array and it being a RAID 5 it will take an entire disk out for the redundant drive and it also sets aside some space for bad blocks and the meta data on the drives.
Hi - I have an MD3000 storage array and am currently considering adding more storage. I have been reviewing the current setup and noted in MDSM that total capcity is 4.2 TB, however the Disk Group (RAID 5 -10 x 450 GB disks inc hot spare) only has about 3.3 TB associated with it. Are we losing this to disk ovehead or is something misconfigured?
MD3000 total capacity query