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Do drives have to be exactly the same size for RAID 1?

The RAID option is greyed out—some research says that the drives need to be the same size, but do they have to be exactly the same size (to the byte) or is there some leeway (maybe a percentage)?

I have two 1TB drives, different manufacturers. One is 953980 Mb and the other is 953740 Mb, so both claim to be 1TB but are 240mb different.

Server specs are:

  • Server Dell PowerEdge T310
  • Running Windows 2008, 64bit
  • Raid Controller Dell SAS 6/iR Adaptor
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up vote 2 down vote accepted

Most RAID controllers probably won't let you use two different sized hard drives. However if it does, you will be limited to the amount of space of your smallest hard drive.

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I don't think this is answering the question though; the point is, both hard drives are 1TB, but when it comes down to bytes, vary only marginally. I wonder if this matters – Dave Jan 3 '13 at 11:13
I will resize the larger drive on the weekend so they are the same and then see if Windows will allow me to mirror. I has assumed that 2 1Tb drives would actually be the same size - we learn these things slowly sometimes! – AusAl Jan 3 '13 at 11:13
Most likely no. – cole Jan 3 '13 at 11:13
@DaveRook from what I've read, they're both 1TB drives - so I highly doubt bytes will matter to a RAID controller. Without knowing what RAID controller is being used (software/hardware) I can merely speculate based on basic functionality of RAID. – cole Jan 3 '13 at 11:15
Okay - answered! Yes, it was due to the minimal size difference. The larger of the 2 drives was the one that had the data on it to be mirrored. It's now synchronising... – AusAl Jan 5 '13 at 19:56

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