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I have a working Linux MD array of five drives.

We are running out of space. The server doesn't have big usage but it does have to be online.

We can't add another controller.

Can I add capacity by buying a USB drive case? Does mdadm allow one to create an array spanning both SATA and USB drives? We don't boot from mdadm.

Are USB drives configured before the mdadm daemon starts?

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It should work; mdadm works at the block device level (it is drive-type agnostic).

Test it on a virtual machine setup first. Set up some lv's to model the raid array, copy the configuration, and then add the usb drive. You'll solve any problems before doing it on the real server.

If mdadm starts before USB drives are configured, you can change the startup sequence.

That said, it's better to use a SATA drive. If you do get the virtual machine copy running maybe that can replace the existing server?

Another thing you might consider is moving your data to an array with bigger disks. If your data is growing you will only have this problem again. Upgrade the server.

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This should operate correctly, but keep in mind that in most RAID configurations your performance is going to be limited by the slowest drive in your array. If you mix USB drives with your existing SATA drives, you may see your performance drop.

Also, note that there is no "mdadm daemon" necessary for things to function. The mdadm tool is typically run at startup to assemble any existing arrays, but you can always run it later on in the startup sequence (or by hand). Actual array handling is in the kernel.

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