Addinn more drive space for my ReadyNAS 2304

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MiC_U wrote:

 

 i thought i can mix drives, no drive size order matter , right ?

 

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No, the order you add them does matter.  If you had inserted 3 TB, then 4TB, then 6 TB you'd have ended up with a 7 TB volume (~6.3 TiB shown on the volume page).  But 3 TB, 6 TB, then 4 TB gives you a 6 TB volume (5.45 TiB).

The general rules for adding a disk with XRAID is that it either 

1. needs to be bigger than any other drive in the array (> 6 TB if you are at 3 TB + 6 TB) or
2. exactly match a disk size that's already in the array (3 TB or 6 TB if you are at 3 TB + 6TB)

In your case, your 4 TB drive is being treated as if it's a 3 TB drive (a consequence of violating the rules above when you added it).  There's no point in trying to add a 2 TB drive (even if it were healthy) - the system can't add it because it's smaller than smallest drive in your array.  The reasons for these limitations are a bit complicated. 

But if you follow the two rules above you should always be ok.  If you can't follow them, to get the full space you need to destroy the volume and create a new one with all disks in place (which is what the factory default does).  

Also, if you follow those rules, then your volume capacity is computed by summing all the disk sizes, and then subtracting the largest.