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I currently have a two bay nas with 2x4TB HDDs in it. My problem is that i need more space. I dont want to buy another 4TB HDD and a 4 Bay NAS or a 8TB HDD to replace one of my 4TB HDDs since both options woould be pretty expensive.

I know that a SATA port can only be connected to one HDD.

So here is my question: Is there a device/controller that i can use to combine two HDDs into one logical drive, that i could then connect to one of the NAS Bays. If the controller emulates a Singe drive, then connecting it via one sata connection should be possible, or am i missing something?

Are there devices that are available for sale, that have that functionality?

If there is not, is there a way to build something like that myself?

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  • What is the make and model of your NAS? You should never do what you're trying to do. If you need to add more drives to the NAS you either need a better NAS or external HDDs over USB.
    – Romen
    Mar 29 at 20:59
  • @Romen Synology DS218. Why dou you say that i should never do that? I dont see any risks besides data loss if one of the drives dies.
    – rogerg
    Mar 29 at 21:32
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    Your NAS has 3 USB ports that will allow extra external drives to be attached over USB. USB 3.0 is faster than a spinning disk so you will not notice any performance difference if the disk isn't being used in a RAID. -- The only way to "combine" two disks into one is with some kind of RAID controller. A RAID controller is a very complicated system that is not cost effective or reasonable to put in the kind of adapter you want. Your Synology already does RAID (properly and reliably) so there's no reason you should need a "nested" RAID controller. (Which would be unreliable and slow)
    – Romen
    Mar 31 at 15:08

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The software of your NAS is could be an issue, but if it allows it use the USB ports.

So even 100mb/s is plenty for each drive. USB 3 is 5000/mbps or 625 mb/s. Thus with an 8 port USB 3 hub you can safely connect 5 or 6 hard drives to each hub with no major performance loss.

I looked up your NAS and the pictures indicated 2x usb ports on back and 1 on the front. That is up to 6 drives * 3 ports or 18 drives.


I have to say though, this is bad long term planning as your will have huge piles of drives, and a cabling nightmare.

There are special drive cases that hold 24 drives each, and you can use a RAID controller and SAS expanders built into the drive chassis. This will allow you to connect 24 drives now. If you get the correct model it will have an uplink port allowing you to daisy chain enclosures for even more expansion. The upper max for a single RAID controller is around 250 drives which should be plenty.

However, this cost even more money, but will avoid a cabling and outlet strip nightmare.

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    In regards to the specific kind of adapter that OP was asking for, I just wanted to emphasize that those "RAID expanders" you mentioned are usually connected to an actual server/NAS using something totally different than SATA. They do not work in the way OP was asking for but they are the right solution to the problem. Expanding the number of drive bays with something connected over Thunderbolt or SAS is how the big servers do it.
    – Romen
    Mar 31 at 21:32

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