Trying to boot El Capitan, High Sierra, and Win7
Generally, when you move Windows, you have to rebuild the BCD file.

Windows generally looks at the contents of a partition in order to determine the partition type. The macOS operating system relies on the type information stored in  the respective partition table. So it is possible Windows could boot even though the partition types are wrong. On the other hand, macOS would not not be able to read the same incorrectly typed partitions.

Each of your drives have two types of partition tables. Windows 7 uses the legacy MBR partition table and macOS uses the Guid Partition Table (GPT). So it is possible for one operating system to see the partitioning differently than the other operating system. 

Fast forward to yesterday. High Sierra has a tendency to reset MBR partition tables to what is referred to as a full protective MBR (PMBR). I layman terms, High Sierra may have erased Windows 7. Normally, this is an easily recoverable event, but I this many not be true in your case. To determine the damage, one would first have some detailed information about your current partitioning.

I am not familiar the Mac Pro model, so I may get some of these commands wrong. None of the commands alter your Mac. The commands just provide information about the partitioning and volume formats. Edit your question and add the output from these commands. If necessary precede a command with sudo.

diskutil  list
sudo  gpt  -r  show  /dev/disk1
sudo  fdisk  /dev/disk1


I believe you have to boot to macOS Recovery to execute these next commands.     

gpt  -r  show  /dev/disk0
fdisk  /dev/disk0