I have two HDD, both are WD Black series, one is 2,5" and the other one is 3,5". Both have 64 MB of cache and 7200 rpm.

The former one is 750 GB, the latter is 1 TB. I would like to create RAID mirror with them. Are there any contraindications? I do not want to spend 100$ on another 1 TB HDD and both are relatively new, about two years. (Yes, I know I'll lose 250 GB).

Best Regards, Kamil

  • What's the OS being used?
    – ewwhite
    Commented Feb 21, 2016 at 14:01
  • It will be Linux. I will have SSD as OS drive, and would like to create RAID for storage. I'll use this motherboard: (gigabyte.pl/products/page/mb/g1sniper_a88xrev_30/specs). It has RAID capabilities built-in, UEFI access to RAID controller, as I understand.
    – kmwil
    Commented Feb 21, 2016 at 14:05

1 Answer 1


No, it's OK to create RAID in Linux using drives with different size. You wrote "mirror", so RAID1 on 2 drives I guess. Size of such RAID may be UP TO size of the smallest drive. I have 2 RAID mirrors on 2 disks for 2 drives same capacity, one for /boot and one for root, rest of disks is not RAID1 but swap area outside of RAID - for better speed.

I see no problem creating Your RAID mirror size <= 750GB, and use rest of 1TB HDD area for temporary data outside of RAID mirror. So You don't even lose Your 250GB, just can't use these for mirror, actually You can, but, You know, by design, in mirror same data written to different disks, so when one drive broken, You still likely have copy on second drive. Now You can easily imagine what happens if You put both mirror copies to the same drive. Answer is yes You can, nothing to worry about.

And last - You can start creating mirror from one drive. Use 750 GB drive to start, or You may not add second drive, if You start with bigger one. 750GB drive cannot be added to 1TB mirror.

P.S. 2.5 or 3.5 - they have different speeds, so when You write to such mirror, You must wait until slowest drive finishes, so RAID mirror write speed will be abut the same as write speed for slower drive of two. For read access and sequential read You can probably achieve read speed slower+read speed faster drive. It depends on Your RAID implementation, some of them are not so smart and have same speed limits as for write.

  • 1
    Thank You, very useful info. I thought that I'll lose that fourth part of my bigger drive, but now I know I can use it outside of RAID. And I didn't know that the order of HDD in RAID creation is important. I had RAID capable motherboards since 2009 I think, but I never used this feature before. On servers at my work this is more simple, we always have arrays, each disk is the same, so when I wanted a simpler backup solution at home, those question arose :)
    – kmwil
    Commented Feb 21, 2016 at 14:50

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