Peter wrote

No. It is not possible.

Yes it is.

First of all you schedule your "backup" nightly, so when you decide
to swap them, it is not the same time as you "left off your C drive".

Want to try that again in english ?

Secondly, your D drive must be invisible to OS when C drive is active,

No it doesnt.

yet be able to start normally when swapped. The only way
to achieve that is to have it powered down except backup.

Wrong. All you actually need to do is never boot the D
drive with the C drive visible. You only need to do that
when the C drive has failed, so that is no big deal at all.

Unless you put a programmable switch on D drive
power line, I do not see how can one achieve that.

Trivial, just have them both powered all the time, dont
boot off the D drive with the C drive visible. Just power
down the C drive manually before booting the D drive.
No big deal since by definition you have to be there
to specify booting off the D drive anyway.