Looking at the chipmaker’s website, there aren’t many differences:
- The “X” version supports configuration of the four PCIe lanes to support up to four devices at once. The lanes can be configured as a single 4-lane device, two 2-lane devices, four 1-lane devices, or a 2-lane device and two 1-lane devices.
- The “X” version can support an external power-delivery controller chip (in case the built-in one is not sufficient for the product).
These differences are important if you’re designing an enclosure, but as a user buying an enclosure, the differences shouldn’t matter.
8TB drives are not common, and they’re very expensive, but they’re also not “super rare”. Some quick web searching found three from major brands (plus several more from no-name brands):
- Micro Center sells one for $750: Inland Performance Plus 8TB 3D TLC NAND PCIe Gen 4 x4 NVMe M.2 Internal SSD
- Corsair sells one for $1000: MP600 PRO LPX 8TB PCIe Gen4 x4 NVMe M.2 SSD
- Sabrent sells one for $1200: Rocket 4 Plus SSD
The only 16TB NVMe drive I could find must be connected to a PCIe slot - not available in an M.2 form factor.
WRT enclosure support, it’s going to be a function of the interface chip
If the enclosure’s interface is Thunderbolt, then there shouldn’t be a problem. TB has built-in support for tunneling PCIe data, and NVMe is based on PCIe. So a chip that blindly tunnels the PCIe data should just work.
A USB enclosure will need a bridge chip that implements PCIe, but that is a well-defined standard. I would expect any non-junk bridge chip to support any size NVMe stick. On the USB side of the interface, the bridge chip needs to support the 64-bit version of the SCSI APIs (which is used by both the USB Mass Storage spec and by the UAS protocol) in order to support devices larger than 2TB, but once that support is provided, there shouldn’t be any practical limit.
That having been said, if the enclosure says it supports 8 or 16 TB, I would believe it unless you find information (e.g. independent testing or brand reputation) that would make you doubt the claim.