HTWingNut

They're using Linux...

Remove Disk 1 and then Disk 2 from the array using SnapRAID's suggestion:

https://www.snapraid.it/faq#remdatadisk

To remove a data disk from the array do:

• Change in the configuration file the related "disk" option to point to an empty directory
• Remove from the configuration file any "content" option pointing to such disk
• Run a "sync" command with the "-E, --force-empty" option: snapraid sync -E

The "-E" option tells at SnapRAID to proceed even when detecting an empty disk. When the "sync" command terminates, remove the "disk" option from the configuration file. Your array is now without any reference to the removed disk.

Then add the new disk as d1 (it's ok to not have a 'd2' entry) with your consolidated data on it.

Then perform a new sync.

It will likely take as long as a fresh sync, so you may just want to start from scratch. At least I would. One time parity generation and you're back in business.

That has been resolved. That warning can be disabled.

Just disable WDDA.

sudo synosetkeyvalue /etc.defaults/synoinfo.conf support_wdda no

I bought a half dozen of those like 3 years ago for $189.99. I guess pricing still hasn't improved.

Up to you. If you need more capacity, start replacing disks.

If you can manage four new disks, start over with RAID 6. this will allow you to run with fewer disks using less power, less heat, less noise. You can lose any two disks in the RAID and still keep rolling. With RAID 10, if you lose two in the same mirrored pair, everything is lost. If you want best redunancy then RAID 60 but Synology doesn't support that, and think it's a bit overkill imho.

RAID 10 with 8x4TB is just 16TB of usable storage. You can get four 16TB or 18TB (or even 20TB) in RAID 6 for 32, 36, or 40TB usable storage and use half the power. And give you four more bays to add more disks as you need them.

They don't exist. SSD pricing at 2TB and smaller are basically on par with 2.5" hard drives now. 4TB drives are also dropping in price considerably.

While it should technically be possible to make 2.5" 4TB 9.5mm thick or 2TB 7mm thick CMR HDD's, there's no point in investing in the equipment to make them.

Consdering the three remaining HDD manufacturers (WD, Seagate, Toshiba) also make SSD's, I would expect SATA SSD's to completely phase out 2.5" HDD's probably within the next year or two.