I found a second internal drive and any external drive still turns off about five minutes after it was last used no matter what settings or tweaks I tried, so I created the following batch script that is named keepalive.bat and placed in my /Users/<name> folder. A companion file that is zero bytes long and named keepalive.txt is also placed there. Lastly, I created a startup.bat script file that is called once at startup, and placed it with the other two. You can run startup.bat manually (run the Command Prompt, then type in C:\Users\<name>\startup.bat to run) or you can add to your Windows startup (google "add to startup windows <x>", where <x> is 7, 8, or 10 depending on your version--just remember to use a shortcut to the startup.bat, not the actual file). The drawback is that you'll have a command prompt icon in the taskbar all the time and a zero-length keepalive.txt file in the root folder of the disk being kept alive, but I can ignore those. If you can't, don't try this. However, if you do use this, you can also open the command prompt icon at any time and ctrl-C to end the batch job if you want to go back to how the disk(s) operated originally. By the way, repeatedly starting and stopping a hard drive is far more wear and tear than leaving it running all the time, just like repeatedly turning on/off a car every five minutes over a long period of time is far worse than letting it idle over that same time period. Lastly, it should be obvious, but you don't need to do this for SSDs since they start/stop instantly.
Comments, which begin with "<--", must be removed from your copy of the file
keepalive.bat
@echo off
:loop
copy keepalive.txt d: <--letter of drive to keep alive, not needed for C:
copy keepalive.txt ... <--for more disks if needed, otherwise remove this line
timeout 60
goto loop
exit
startup.bat
@echo off
start "" /min "C:\Windows\System32\taskmgr.exe" <--another app I start at login
start "" /min "keepalive.bat" <--here's the initial call to the keepalive.bat script
exit