Good day,
Actually, there is always the simplest option which is install Haiku onto an external USB HD and tell the BIOS do default boot to External USB Device. That way no need to touch any GRUB or whatever.
Then again, I’ve found that running Haiku off an USB pendrive causes temporary lockdowns due to presumed write/read cycles at the same time making Haiku not responsive for some seconds. Not sure if due to pendrive not being UASP compatible, or whatever, as running Linux off the same pendrive does not present such issue, or at least doesn’t seem to be present.
Anyway, with an USB 3.1 or 3.2 external drive connected to an USB 3.1 or 3.2 port should be fast enough, being transfer rates at 6 to 10 Gbps, and SSDs around 560 MBps.
Actually, that is what I plan to do: remove the multiboot and boot each OS from its external drive when needed.
Regards,
RR