Dual Booting Windows 10 With Windows 7 - No Drive Seen

Have a look at this YouTube page, I believe this is the issue you are describing. More information is also on this page.

The two links you sent doesn't really apply to me, since I already have Windows 10 running, but need to dual-boot Windows 7 to it.

Thank you for the links.

Is it possible that you need to install the IRST driver? is your BIOS SATA Set to RAID? if so then the install media won't see that partition as an installable volume until it has the IRST driver installed. Also, I'm not sure that you can install multiple OS on a Windows 10 OS disk? Usually, that is done on a separate Disk/SSD. another option that you have is to run Windows 7 64 bit in a VM Depending on how much you actually need to use the Win7 OS this could be a viable solution.

Maybe I got the wrong drivers, but the driver was not compatible.

The Dell BIOS comes as RAID as default. Nothing else works for the T5810 Workstation.

Yes multiple Windows OSs can be installed on same disk but on on separate partition. Currently, I have on my old laptop, Windows 10 and XP.

Interesting you brought up VM. Using VirtualBox (my favorite), for the past two weeks, I cannot install either Windows 7bit or XP 64bit on the virtual machine. I tried using the Windows ISO and the VMDK, followed the instructions precisely, neither of them will install. After many tries and a week later, I tried 32bit, both 7 and XP installed. Strange! Since I was just playing around, I removed both OSs to redo them, now a XP 32bit won't install. I also worked on installing MacOS Catalina, that won't work.

Something strange is not to par with my PC with it comes to installing OSs.

I am assuming that since your dual-booting many OS that you understand that when installing a new OS in RAID mode you are required to Install the RST driver before you can see the volume to install too?

When you get to that point press Shift +F10 which will bring up a command prompt where you browse to the RST driver to install? once that RST driver is installed your Volume should show up as a volume to install too.

I haven't actually set up a dual boot since I ran 7 pro 64  bit  and 8.1 Pro 64 bit and then they were each on their own SSD. I think it can be difficult to see up multiple OS on a Disk/SSD that is already Windows 10 64 bit.  I haven't personally tried. As per the Windows 10 forum I participate in. here is a link to that forum and a thread on the subject perhaps you'll find some answers there. Dual boot Win10 Win7 Solved - Windows 10 Forums (tenforums.com)

It's because you have enabled UEFI only and the Windows 7 ISO that you got Legacy only.
Download a ISO which support UEFI and installation on GPT partition. Many Windows 7 ISO don't support efi and only supports MBR.

There may be problems with an nvme SSD boot disk invisible after a Windows10 installation with UEFI boot was destroyed by incorrect writing (when installing different OS, or Windows in legacy boot mode).

It is hard to tell, who is the culprit, DELL's own bios, or Intel's disk controller BIOS module...

But for now on the only option is getting a low-level disk-writing software and zero-erasing the disk, at least it's first sectors.  If you don't have a working workstation with a spare SATA port, try linux live-boot media.

P.S. For those who believe, windows 7 can not boot in UEFI.

It is not so. The latest build can be installed with UEFI boot on a disk preconverted to GPT. It can be done with boot tools coming with Windows7 before the installation.

Correct Windows 7 SP1 supports 64bit windows therefore also supports UEFI Boot.  There should be no issues with Dual boot Windows 10 64 bit UEFI and Windows 7 SP1 64bit UEFI. I believe MS has deprecated windows 10 32 Bit completely who would want to use Windows 10 in legacy mode?