"Balancing" a NAS?

I have a 4-drive Synology NAS. And I’m getting close to filling it. So it’s probably time to switch out the drives for larger ones. I currently have four 4TB drives in it.

Does one need to “balance” the drives in the NAS? Like, could I have a 12TB drive and three 4TBs?

It’s a bit complicated, but the docs help some.

Edit: this might be a better article:


I believe if you have a set of 12,4,4,4 then the effective usable size of the 12 will be 4 (in a RAID configuration).

Be sure to check all questions with Synology support before making any changes. Synology devices are feisty little demons.


Which RAID configuration are you running?

You’ll end up with a lot of unused space. SHR is better with mixed sizes, RAID5 the worst (the smallest drive will “overrule” bigger drives).

With just one 12TB drive and 3 4TB drives, this will happen:

You would be wasting 8TB on both SHR and RAID5.

If you add another 12TB (12,12,4,4):

SHR will fully use the capacity, with RAID5 you are still wasting a lot of space.

Check different combinations with the Synology RAID calculator.

If you’re not using SHR or RAID at all and just using them as single disks, there isn’t an issue just using one drive at 12TB and the rest at 4TB. I just switched mine over Christmas from a single 8TB and 3TB drive to an SHR volume over 2 x 8TB drives.

Reminder to have a backup. If they are SHR, then it can take a long time to replace one drive at a time. I ended up buying an 8 bay synology that I could start fresh and copy files over.

I’ve an external drive hooked up for a Hyper Backup, it’s backed up to C2 and I take a copy when I go in to work via SFTP, so I’m fairly well covered (I hope!)

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I’m on the hybrid RAID (SHR)

Then you can just buy two new drives and that will expand your storage partially. You can upgrade the other two down the road. Synology’s RAID calculator will give you an idea what the capacity will be with a variety of different hard drive sizes.

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Thank you so much everyone! @tonytx05, @Lars

I think two 12TB drives is do-able with the goal to get to four 12TBs over time.


Be sure to follow the directions on the synology website. and note

If you plan on replacing multiple drives, you must be careful to replace drives one-by-one.

Also, depending on many factors, the rebuild of your volume can take from 6 hours to days. Patience will be key.