Thanks for the prompt reply.
1)  So it should be no lasting problems, if I just SWITCH with the BIOS whatever drive will be booting 1st? And switch again?
2)  And the 2nd installation, on the 2nd drive, shouldn't destroy anything from the older installation (on the C drive, which I want to keep)?
(both of the OS are the same Win 9
3) I understood that Win 95 does "protest" against Win 98 even on the other drive?
4)  If I disable C drive with Win 95, install Win 98 (or Linux) on D drive, and again enable drive C, will it be OK?

Thanks again