I played a bit more with this. The problem must lie in windows caching, switched USB drive to "optimize for performance" and the tasks
-install syslinux BS
-get the syslinux BS and copy it to the same drive
-write back original BS
using grubinst.exe became impossible, even with the help of touchdrv.exe, which works brilliant when only grub4dos BS is installed, thanks.
Run in all kind of variants, before each line, just after writes, just after grubinst.exe writes, with 5-10 second pauses etc. File copy was moved at the end- no joy. The original BS was not written back.
So here came the light:syslinux.exe -f j: j:\syslin.datSyslinux 3.63.
This writes the syslinux BS to syslin.dat. BS in drive J: is not touched. If file is omitted, then BS will go to the partition. This is not in SysLinux documentation, neither in their site, nor in the docs, coming with the package, which I checked so many times for something similar. Downloaded the latest 3.70- still not a word that it can write it's BS to a file, instead to a partition.
Perhaps I can add a new function to grubinst. Instead of writing directly to disk, it saves the bs to an output file. Then, you can use external tool to save the bs to specific drive whenever you feel right. This should fix the problem with syslinux.