Subject: Re: Very OT: Help with Mac-Win problem?
GUID/GPT and MBR ... It could make a difference which OS was used to format the partition ...
So maybe format it again using the macOS Disk Utility, ensuring you use the MBR partition scheme?

Thanks (to oddhack too), but using GUID or MBR is a decision you have to make right at the beginning as it's not about how one single partition is used, but about the whole disc's partition scheme. So all partitions would need to be deleted, dual boot and all partitions with their contents recreated again.

Btw: I regret and apologize for posting in desperation this far too OT question and hope it does not set a bad precedent. I suggest who has another tip sends it via private message ("Privately email Said only" underneath the posting text box).