# External Storage - One versus two drives



## rwde (Apr 15, 2016)

Planning some external hard drive storage - is there any advantage to a 2 drive array (2 x 2 TB drives) versus a single 4 TB drive?  This will be an external hard drive(s), connected with USB 3 for primary storage of photos (and will be backed up either to NAS or cloud).  Don't think I need RAID on the primary external drives, but do I get any meaningful advantage or disadvantage from 2 drives versus one?

I'm looking at the WD My Book or similar, if anyone has thoughts in that regard.


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## tspear (Apr 15, 2016)

Often the two drive solutions are faster. But not always, compare the specs. Specifically you want to check the main factors:
Bus Speed
Cache Size (if available)
Drive seek time
Disk speeds
Disk Speed


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## LouieSherwin (Apr 15, 2016)

Lightroom does not as far as I know try to take advantage of multiple drives for image storage so have two I would say that having two smaller drives rather than one larger one is only going to be more of a headache than a help. There are some performance gains when you have your catalog and caches on a separate drive from your images. So putting your catalog on your system drive and the images on the external drive will help.

Another alternative that would offer higher reliability is to get a 2 drive system with 4 GB drives that implements RAID 1 (mirroring). WD and others offer external cases that provide this function. This protects against failure of either one of the drives but does not replace a good backup implementation. But in the case of a drive failure your recovery is instantaneous and very little if any data loss. Whereas with just a single drive and good backups your potential data loss is back to your last good backup. You just have to decide the amount of risk you want to take.

-louie


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## clee01l (Apr 15, 2016)

RAID Drives left a bad taste in my mouth.  For every RAID drive there is a RAID Controller (hardware) and usually a Proprietary filesystem. If the RAID controller fails, the proprietary filesystem will prevent you from (easily) reading the data on either of the drives. If the data is striped, then parts of each file reside on both drives It the RAID controller fails, then you need to replace it with a new one before you can recover your data.  If your RAID is mirrored, then you have redundant copies on each disk So there is protection if one of the disks goes bad.  However if your RAID is using a a proprietary filesystem, then you need specific software that can read that filesystem to recover your data..  FWIW, all of the above happened to me with a NAS RAID drive.   Since then I have simply bought a single larger capacity drive for my backup and external storage needs. 

 I have a WD 2X2 TB My Book.  It is configured as a single 4TB logical volume and HFS+.  Since Apple took the software RAID option out of OS X, I'm not sure what I will have to do when this drive gets flakey


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## LouieSherwin (Apr 15, 2016)

Cletus, good point regarding RAID controllers. I forget thatsince like you I have been using the software RAID in MacOS to mirror two internal drives  in my Mac Pro 2012. I started using mirroring when I needed to upgrade my 1TB internal drive. The 3TB drives and the were brand new at the time and I didn't trust the reliability of the technology. 

FYI Apple has not removed software RAID from the OS, just from the Disk Utility app. Software RAID is still available from the command line diskutil. And you can still grab the Yosemite Disk Utility app and run it on your El Capitan system if you want to.

-louie


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## LouieSherwin (Apr 15, 2016)

I had some additional thoughts to share. Cletus raises the concern that data stored by a RAID system in not accessible except through a functioning RAID controller. This true for most flavors of RAID. However, it is probably not true RAID 1 (mirroring). The definition of RAID 1 is a block for block duplication at the disk level. This means that you not only duplicate that data but also the format of the disks. So in practice this means that you can break a RAID 1 mirror and mount either of the resulting volumes on your system. 

If using a HW controller in an external case and the controller has failed on some fashion it may mean that you will have to move one of the mirrored disks to a new enclosure with a working interface USB, Firewire or eSATA to connect to your system. But because the data and the format are maintained by the mirror you will be able to access all your files as before the controller broke so long as you can connect the HD SATA interface to your system.

Now I suppose that a particular implementation either HW or SW could mess this up but that would contrary to the specification. However, if your drive enclosure states that it supports RAID 1 than I would inclined take it to mean that it is working as described. I know this to be true of my SW RAID 1 on my Mac form actual experience of breaking the RAID and mounting one of the drives externally to copy back to a larger RAID 1 pair. If you wanted to be absolutely certain of this for a HW RAID box you would have to remove one of the mirrored drives and make sure that it could be mounted independently.

Just to be clear to everyone reading this, no flavor of RAID takes the place of a carful and consistent backup procedure. At a minimum an automated periodic local backup. And to be really safe an additional automated offsite backup, easily accomplished these days with one of the several cloud backup services. There are fairly frequent requests here where someone's entire image library has been permanently lost due to HD crash or accidental deletion and there was never any backups made. 

-louie


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## rwde (Apr 15, 2016)

Appreciate the information.  I had my own problem with  HP device that was RAID 1 but in some proprietary format. I learned more about data recovery than I ever wanted, but eventually found recovery software that worked. I have NAS on a Synology device as backup - so the drives that I'm adding are pure data storage for image files. My inclination is to keep it simple, reasonably fast, and backed up.


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## clee01l (Apr 16, 2016)

LouieSherwin said:


> ...If using a HW controller in an external case and the controller has failed on some fashion it may mean that you will have to move one of the mirrored disks to a new enclosure with a working interface USB, Firewire or eSATA to connect to your system. But because the data and the format are maintained by the mirror you will be able to access all your files as before the controller broke so long as you can connect the HD SATA interface to your system.


 If the RAID drive is formatted with a proprietary filesystem as many are, you still need to read the data on the disk.    This was my dilemma when my RAID1 RAID controller failed.  I was fortunate as I had two 2X1TB NAS enclosures by Buffalo.   When The RAID Controller failed on one, I was able to swap out the Disks with the remaining good unit and extract the data off of the drives that came out of the bad box.  I also lost confidence in the remaining RAID enclosure for if it failed like the first I had no means of getting the data off its disks.


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