On normal Bios, not UEFI, with two HDD installation goes like this.
Both hdd should be connected to the system
install Linux on the second hdd (where you want) with Grub
restart machine. Windows will boot now. Examine is everything ok with Windows
restart once again, go to BIOS, select second hdd, where is Linux installed, to be first for booting
When Linux start and you logon on it, just do " sudo update-grub" and watch if grab added windows loader to grub.cfg
After that you should have dual/multi boot Windows/Linux on separate hdd.
Problems could be with uefi, ssd, Windows 10 power options
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