DSM 7.1 Can't expand SHR1 with 4th HDD smaller than rest?

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DSM 7.1 Can't expand SHR1 with 4th HDD smaller than rest?

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I have 3 16TB HDDs in my DS920+ that I just configured in a storage pool with SHR1. From the guidance here, it seems I cannot add an old 12TB HDD to the pool. Specifically:
"The drive you want to add must be equal to or larger than the largest drive in the storage pool, or equal to any of the drives in the storage pool."

Trying to understand why is this case and if the article above is accurate. I read how SHR1 works but can't think why this would be disallowed.

Can I expand my pool with any HDD that is not 16TB in size without wasting space on it? If I can only expand with a 16TB drive, should I change over to RAID 5 instead of SHR1 in my situation? Pros and cons of doing this?

Trying to understand why is this case and if the article above is accurat
You cannot add a smaller drive to an SHR pool. You can create a new pool with that drive. Details for adding drives, are discussed in the Synology Knowledge Center.
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Pros and cons of doing this?
SHR vs conventional raid will be off the bat that you will not be able to combine drives of different sizes to maximize space (following the rules that you found on SHR and as Telos mentioned as well). With RAID you would waste space when it comes to mixing drives, but then again if you for example one day start moving up to some higher-end units (RS one) you will notice that some of them DO NOT support SHR, only RAID#, so that's one more thing to consider.

RAID would be best to be used with drives of the same capacity in order to capitalize on max capacity and in some cases speed as well.

Atm you have several options. Start fresh, 12+16 drives combined will work in that case as it would be a fresh start, or maybe create a new volume with a smaller drive.
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And why this is the case:
easiest is to consider a 16TB and a 12TB.
Mirror is only possible when you start with the 12 TB and add the 16TB later, creating a redundant space of 12TB and 4TB unused. The other way around is not possible.
The same principle but more complex is the case with the 3 disk case. Add a smaller one, and the maths are too difficult to solve without redoing a full format and start with the smaller disk.
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