Synology Hybrid RAID Technology
The so-called Synology "Hybrid RAID" (SHR) technology is a simple technology developed by Synology to prevent wastage of disk space when disks of different capacities are installed to the NAS. Briefly, this technology allows using all or practically all available disk space, keeping the data stored safely with redundancy.
How does it work?
Assume a Synology NAS has four disks installed: two are 2TB and two are 4TB. In this case the NAS creates "classic" RAID5 using 4 x 2TB disk space. For classic RAID5, two disk tails (2x2TB) will remain "wasted space". In case of a "Hybrid RAID" these tails will be built into RAID1 (mirror) – another RAID with single component redundancy. Both RAIDs will be automatically "spanned" to a single storage. So instead of 6TB (of classic RAID5) the NAS will provide 8TB of disk space, but with reliable single disk redundancy for data protection in case of any single disk failure.
What could be the problem there?
As a "Hybrid RAID" uses "classic" RAIDs under it, the possible problems with this type of RAID are the same as with "classic" RAIDs: the failure of more disk components than redundancy allows, RAID configuration metadata damage and so on. The only possible "complication" in comparison to a classic RAID - more than one RAID configuration can be combined (spanned) with the "main" one.
What can be done?
If the disks are of the same capacity, a "Hybrid RAID" is a "classic" RAID that can be recovered just like any "classic" RAID with any RAID recovery software.
If two or more drives have larger capacity, tails of these disks must be assembled to another RAID and then spanned with the main RAID.
So a "Hybrid RAID" can be recovered just like a "classic" one. The only thing to pay attention to is the correct assembly of possible "tail" components (using correct offsets) and correct spanning them with the base RAID.