EasyBCD is a software utility for Windows that lets you control and configure the BCD/BOOTMGR bootloader for Windows 7, Windows 10, and Windows 11 for dual-boots.
neosmart.net
Or, just interrupt the boot process when you turn it on, and select the relevant drive.
Or, turn the Win 7 things into VirtualMachines. Accessible at any time.
If the other drives were attached when you installed WIn 10 it should have added itself to the win 7 boot loader and you should get a choice at startup still?
When the installation is finished, you’ll see a boot menu every time you boot your computer. Use this menu to select the version of Windows you want to boot.
I tried installing Windows 10 a few months ago, I had 2 x Win 7 Hard Drives and I installed Win10 qned it screwed up BOTH the Win 7 drives., This time I got clever. I disconnected the other three Win7 drives and installed Win10 on a nre blank hard drive. I now have a Win10 Hard drive and 3 x Win7 drives. They are NOT all on the same drive. Each drive is a different installation.
EasyBCD is a software utility for Windows that lets you control and configure the BCD/BOOTMGR bootloader for Windows 7, Windows 10, and Windows 11 for dual-boots.
neosmart.net
Or, just interrupt the boot process when you turn it on, and select the relevant drive.
Or, turn the Win 7 things into VirtualMachines. Accessible at any time.
F11 or F12 are the two most common keys to select the Boot device, the option is shown at the bottom of the screen, usually, at the same time the key to press to enter setup is shown(usually the Delete key).
This may depend on what you have set for the boot drive in the UEFI, I have mine set to a drive and can use F11 to boot my w10 drive when needed; otherwise it boots right to W7 without me doing anything extra.