I think you may find that some systems (UNIXs?) probably want to be booted from unit 0. So it might be good to reserve that until it's absolutely known to be required.
Another thing to know is that both RT and RSX can boot from alternate controllers if the OS is SYSGEN'd with an additional controller in it. (probably RSTS too) If you like, I can check to see if the RSX BASELINE already includes one. (seems to me the M+ BL did customarily)
In order for a disk-based OS system image to be bootable and fully functional on a particular volume, the OS must have been generated to include that disk controller. For example, an RSX system with both RL and MSCP controllers can be booted - when copied to either type of drive. If the that system image is copied to an RK disk, it would not be bootable unless the image also contained an RK controller as well. This also extends to controllers at "Alternate Addresses" - which must be explicitly included, by address and type, at SYSGEN time.
I hope that's clear. It's implicit really - but sometimes we don't consider it ahead of time.
