Mac OS X Sierra 10.12.2

Have updated from Mac OS 10.12.1 to 10.12.2. System has an external Promise Pegasus 2 6 drive sub-system.

System was using the external drive as the "/ " (root) drive and not using the internal drive at all.

Under 10.12.1 and earlier releases of OS X booting to any external device worked.

Be aware....10.12.2 no longer even "sees" the external drive at boot time. The Option key does not show the external drive at all.

The external drive is available as a mounted device after boot.

It is possible to re-install the OS to the Promise system, but alas, it will not boot to it.

Apple tech support says that they do not support "/" devices as external drives.

Since when? I have several system setup this way for fault tolerance (using raid) which is not provided by the internal hard drive.

Have not found a work-around.

Any ideas on how to correct this deficiency?

Thanks.