dell 990 ssd window 7 install

SSD's are advanced format and require AHCI setting for sata operation.  If you do not use F6 driver windows will have STOP 7B error and or will not "SEE" any hard drive to install on.  F6 drivers are required for advanced format. XP used floppy, win7 and up use F6 drivers for AHCI/RAID disk using USB thumbdrive only.  Windows 7 will and up will also state"A required CD/DVD drive device driver is missing"

This error can occur if one or more of the following scenarios are true: 

• The CD/DVD drive may not be compatible with Windows 7 or Windows Vista
• The CD/DVD media may be damaged or dirty
• Your CD/DVD drive may be incompatible with the Microsoft AHCI driver (msahci.sys)

https://www.dell.com/support/article/us/en/19/sln291835/intel-rapid-storage-f6-drivers-for-windows-7-8-8-1-and-10?lang=en

After you select the install now option, select the Drive Options - Advanced menu, then select the Load Driver option.

F6 ADVANCED FORMAT

https://www.intel.com/content/www/us/en/support/articles/000005611/technologies.html

when i try to switch it says uefi file system not found ? so i guess leave it set to Legacy?

You cannot Downgrade windows 8 or 10 to windows 7.

AHCI is SATA OPERATION Mode and has nothing to do with Secure Boot or UEFI.  This is under SYSTEM CONFIGURATION not the GENERAL Settings.  Windows 7 will need to be installed via DVD and F12 booting.

USB installs will not work here.

You will have to boot a windows PE media and run diskpart to CLEAN the drive. You CANNOT INSTALL 7 over the top of an existing windows 8 or 10 install. Windows 7 does not have UEFI secure boot.

Secure Boot must be off and Legacy option roms ON.