I have a 1 TB Western Digital external hard drive that is old but works fine, I also have a 4 TB WD RAID (2 x 2 TB hard drives in one casing) that stopped working which I'm trying to recover. I must say it turns on when I plug it, but mac doesn't recognize it.

I pulled out HDD #1 from the 4 TB RAID, then connected it via the SATA connection to the housing of the 1 TB. Physically, it connected without a problem, but my Mac doesnt recognize the "new" drive.

Any ideas what I should do in order to see the data from the 2 TB HDD?

  • Which RAID level did you use to combine the 2 2TB drives in the RAID casing?
    – nohillside
    Feb 29, 2020 at 14:37
  • I don't remember setting it in a specific way when I purchased it so I am not really sure... is there a way to find out? I know that one HD is a copy of the other... if that helps.
    – Zozo
    Feb 29, 2020 at 15:06
  • How did the RAID system show up in the Mac, as a 2 TB drive or as a 4 TB drive?
    – nohillside
    Feb 29, 2020 at 15:09
  • When looking at Disc Utility, under the 'external' to the left I see the HD, its named" "2TB RAID GRAFIX". This is the name iI gave it. Does it help?
    – Zozo
    Feb 29, 2020 at 15:14
  • Is this for the 4 TB WD RAID or for the #1 disk you took out of it? We need the size the 4 TB WD RAID shows you in Disk Utility. As Ipacheco wrote in the answer: If it shows up as 4 TB each disk alone is useless, if it shows as 2 TB you could try accessing #2 instead of #1.
    – nohillside
    Feb 29, 2020 at 15:18

1 Answer 1


If your RAID is 4 TB from 2 x 2 TB then it is RAID 0, a stripe set where one 2 TB drive is an "extension" of the other. Each 2 TB drive won't work by itself. If any of them fails, the other is useless. This setup is useful for write and read speed, but it decreases confidence in the set.

So, if you pull any of the 2 TB drives from the set and tries to use it, it won't work. You need to repartition the drive as a single drive (no RAID), but then the other drive will be useless forever (you destroyed on of the drives in the set.

If it was RAID 1 (mirror), you would have 2 TB from the 2 x 2 TB set, but each drive would be an exact copy of the other and you should be able to use each one by itself. This setup seems a waste of space but it increases confidence in the set since any 1 drive failing would still allow you to access your data.

  • I am pretty sure I chose the mirror option. After cheking the Disk Utility, I see the the mac recognizes the external HD, yet its grayed out. I cannot mount it and can't use First Aid. Any ideas about how to proceed? Thanks!
    – Zozo
    Feb 29, 2020 at 15:10
  • @Zozo If you did choose the mirror option when you set up the RAID system, then each of the 2TB drives will have the same content, and be mountable and usable by itself. To test, remove both drives from the set and one at a time, connect one 2TB drive to the Mac. If one of the 2TB drives is good, that drive will mount. If neither of the drives mount, then either a) both 2TB drives have failed, or b) you inadvertently set up the RAID system as RAID 0, and each drive (as Ipacheco explains above) is now unusable, or c) something (electronic bits or cables) in the RAID enclosure has failed. Feb 29, 2020 at 17:56
  • DavidSupportsMonica, I appreciate your detailed explanation. I was able to connect each HDs, they show up, I get to see their size (2Tb each) but they are greyed out, so I cannot mount them and access the files. Any ideas about how to continue from here? Thanks in advance!
    – Zozo
    Feb 29, 2020 at 19:29
  • @Zozo Did you connect each 2TB HD separately to the Mac? If you did, and each appears on the Mac but is greyed out, then I think either you inadvertently set up the system as RAID 0, or circuitry in the RAID enclosure or cable(s) has failed. I don't know if commercial or sophisticated data recovery services might be able to recover your data from the HDs. You could try to reformat the disks, but that would surely destroy their content. Mar 1, 2020 at 10:06
  • DavidSupportsMonica, thanks for the info. I connected HD separately to the mac. Then, just to make sure, I got both of the HDs and placed them in a different casing of another RAID drive. I am getting the same results in all - both HDs are grayed out. I am curious about learning more about the first option you suggested - inadvertently changing the RAID setup to 0. The HDs worked well for about 3 years before this happened and the mac stopped recognizing them out of a sudden. I haven't touched the settings. Wondering if there's still a way to recover the data... any other ideas?
    – Zozo
    Mar 1, 2020 at 14:37

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