Need some help with older Nas which only allow 16TB as largest volume

[apvm]

I have two older NAS, one is Synology 413 and the other is Asustor AS1004T, i upgraded them both with 6TB drives lately and found following difference.
Synology 413 volume size has increased to about 15.66TB but Asustor AS1004T only to about 11.22TB both are 4X6TB HDD with Raid 5 (Synonlogy is SDH).
Wondering what can I do or nothing I could do with AS1004T to its max volume which is also 16TB. Thanks.

 

[Tech Junky]

Downloads | ASUSTOR NAS

Downloads,ADM

www.asustor.com

Release Notes - ASUSTOR NAS

Release Notes

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First thing I would do is upgrade the OS and see if there's an update that allows for more storage. I find it odd that these NAS devices limit the capacity in the OS vs the drive sizes. Another reason to just DIY a file server instead of off the shelf setups.