746 Gb=3TB disk

alfinmass the ARM devices such as the Duo v2 (x8x8x has one of these) have always had GPT support so it should've recognised the full capacity of the disk immediately on that. Though sometimes if disks have been used previously (e.g. in a PC) they might not be recognised properly due to the way they've been formatted.

For the x86 devices like the Ultra 6 Plus GPT support was added in 4.2.16. So it sounds like your NAS came with 4.2.15 or earlier. Now that you've updated the firmware to 4.2.21 you could do a factory default with all disks installed if you like. This would probably be quickest.

Do note that the NAS like most PCs measures using TiB (i.e. 1 TiB = 1024^4 Bytes) whereas disk manufacturers use TB (i.e. 1TiB = 1000^4 Bytes). However like with PCs, there isn't a distinction in the naming between TiB and TB, so TB is used. So 3TB (disk manufacturers measurement) = 3000 / 1024 ^ 3 * 1000 ^ 4 = 2794GB (NAS/PC measurement). There is a 4GB partition dedicated to the OS.

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