Hi, cause I did not find documentation on this issue I will publish some experiences here.
After years of excellent functioning I had some problems with one of my two 910HD receivers (a minor defect in the usb-wifi stick - but repairing the one problem the other [cccam] problem came - that's how it goes, everybody knows).
The receiver-to-repair was still running on Hyperion V2 so I decided to install V4 on another partition. Install on HDD-partition, or USB-partition is easy as pie with the install-manager of Hyperion. But once installed, I could not start-up on the new partition. Pressing the number-buttons on the remote control on booting had no effect.
I thought somehow my multiboot ór my Uboot was defect. I tried everything. E.g. copied the environment from the working and identical 910HD (with Bootplus v2.3 - save, load option). And ended up with only the 'booting' message on the frontpanel, and nothing else happening (because the new bootsettings directed to a partition without a working image on it).
Happily Uboot was still working - do I builded up my box from the beginning. Starting with an old working 'all' flash image. Finally got it working again - but still no multiboot...
Well, then I tried something stupid: exchanged the batteries of my RC. RC was always working fine on switching channels but maybe... And it worked. The whole problem was that for multiboot somehow there seems more infrared signal from RC needed.
However, I have a perfectly new installed 910HD here, now. Everything working more then fine.
And now it is documented on the internet: When Multiboot not responding to your RC, always first change batteries!!!
PS, is there a way to check which version of Uboot is running?