Netgear Readynas NVX (Buisness)

Booting from the internal drive is not an option.  While it is sometimes said that the ReadyNAS "boots from the drives", that's not entirely true.  It boots from a slimmed down version of the OS on the internal flash and in RAM  and then chain loads the OS on the drives.   My Linux skills are insufficient to tell you how to accomplish that with an alternate OS other than in general terms.  The internal flash behaves as a USB drive, so you'd use syslinux (as Netgear has) or GRUB (I don't know any reason it won't work) to do the chain-load.   The trick is getting a slimmed-down enough system on flash to accomplish it.  Using a compressed file with the RAM drive content is one way Netgear accomplished that.

If it helps, normal boot is defined in syslinux.cfg on flash this way:

label Normal
kernel kernel
append initrd=initrd.gz reason=normal

So, it extracts the contents of initrd.gz to RAM as the system disk and uses the kernel that's in the flash memory.  But the "magic" is obviously in the content of initrd.gz (which, if memory serves, is really something other than a gzip file).  I really thought I had a file extraction of a 4.2.x initrd.gz, but I can't locate it.  All I have is an OS6 one.  My recollection, too, is that they do not use chain.c32, which Google tells me would be the normal way.  So they must have included the function of chain.c32 in one of their own binaries.   I believe that the kernel in flash is actually the same kernel that's the operating one.  Since it's going to need all the same device drivers, it makes sense.

One thing to consider is that Netgear turns the internal flash off after boot.  Since you're not going to know how to do that, it's going to stay on all the time with an alternate OS.  I have no idea what, if anything, that will do to the life of the device.  I think Netgear's main reason for turning it off is to make it impossible for it to be inadvertently written to, not for life, so it likely doesn't matter as long as you're sure nothing is writing to it  since the number of write cycles typically is a limitation.

Hopefully, I've given you enough ammunition to Google away and figure something out.  I've not found anything showing how anyone else has used this method.  Everyone seems to simply boot from USB when using an alternate OS.  But that does mean you have to be present when it re-boots.  Perhaps some hybrid of these methods would work best:  put Syslinux  or GRUB in flash but set it to boot from USB so you have more space to work with.