Also, will it allow mixed sized drives? It comes with 2X 1TB HDDs. I have two 2tb wd reds to put into the 2 spare bays. Want to use RAID 5 for all 4 disks. Is this possible?

I may be a bit confused, so let's pause for a moment. Are you talking about installing WD Blue 2TB M.2 drives onto the BOSS, and then trying to create a RAID 5 from them and some standard drives? If that is the case, then this won't be possible. The PERC will require any RAID volumes created there to be attached to the PERC, and the BOSS won't be. You could probably do this at the software level with something like LVM, but I would recommend against it. Best case scenario would be sacrificing all the benefits of having the higher speed M.2 drives in the first place. 

If you're speaking more generally about whether or not the T140 would let you do that with full size drives, then it's a bit different of a conversation. You could absolutely create a RAID 5 volume from 1TB and 2TB drives. The PERC would use the full space of the 1TB drives, and half of the 2TB drives. This would leave you with some wasted space, but you could create another array on those volumes. This would create what is called a "sliced array." We generally recommend against these, because errors or failures can have a more damaging impact. 

I think it would be more helpful to know what you're looking to achieve and what you're working with, so that we can talk about the best way to get there.

Hi, thanks for your response. It's the H330.

Hi, So the os will be deployed on the M2 ssds via Boss card on RAID 1. The server comes with 2x 1TB HDDs drives pre-installed. I want to add 2 extra drives into the 2 spare bays. But these are 2TB drives and the sales chap said that different sized drives cant be mixed. I am looking to have all 4 disks in Raid 5 just for storage. I want to keep the OS and storage separate.

Ok cool, that makes a lot more sense, and is what I thought you wanted to do, but wanted to confirm when I saw that WD made M.2s in that size.

What PERC are the standard drives attached to? If you're unsure, you're welcome to PM me the tag and I'll check it out. Most PERCs can mix drives, like I said above, but I would like to know exactly what you're working with and get you confirmation. 

With that being the case, you should be able to create the RAID volume among those disks. You can reuse the extra TB on the other drives, if needed. Just keep in mind that if there is ever an issue with the RAID 5 volume, it can affect other volumes that share physical disks. If this is a home server, or a non-critical system, then it probably isn't that big of an issue.

Hi, it is a home server/business use. In that case, would you recommend RAID 1 across all 4 disks instead of Raid 5?

Appreciate your help.