Dell Precision 7520 - hard drive not visible when booting into Linux from USB

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@RichardWattCV  I definitely wouldn’t switch to Legacy boot.  Keep UEFI boot, especially because Legacy vs. UEFI won’t affect the visibility of your storage devices. As for Secure Boot, some Linux distros now support Secure Boot, so if you can boot yours with Secure Boot enabled, then keep it enabled because it’s a nice anti-rootkit mechanism. If not, then disable it.

But the reason your storage likely isn’t visible is because your system is in RAID mode rather than AHCI. Even if you don’t have an actual RAID virtual disk configured, having the system in RAID mode causes the Intel Rapid Storage controller to abstract the storage devices from the OS, which means you need a suitable driver for the Rapid Storage controller, and there might not be one for Linux. Switching to AHCI will expose the storage devices directly to the OS. But be warned that switching between RAID and AHCI on Windows systems is something that’s typically meant to be done prior to (re-)installing Windows. If you switch that setting on an existing Windows installation and want to keep running in that mode, that Windows environment will become unbootable until you either switch the setting back or jump through some steps to tweak your existing Windows installation accordingly. I’ve read, but have never personally confirmed, that booting into Safe Mode AFTER making the switch will succeed, and just allowing the OS to start in Safe Mode is enough to give Windows the chance to update itself. After that, you can simply reboot normally and should be good to go on Windows after the switch.

If your bootable environment can’t read NTFS volumes, that’s a whole separate issue. The steps I outlined above would likely resolve a case where you can’t see the actual storage device itself.

I just checked the settings in the manual and, RAID is the default option - I'm wondering why a system with a single hard drive that has 2 partitions would be configured to run in RAID mode because it's not acting as RAID 0 or RAID 1.

The Linux version on this recovery USB flash drive is Knoppix-based from 2017 and it does launch the Knoppix environment but it doesn't see the HDD and the software tools.

So, to change to AHCI, would I just do that in the BIOS first (making a note of what it says in the Drives options) and then boot with the USB flash drive attached, do what I need to do in the Linux environment, restart to get back into the BIOS and put the setting back to RAID On?

@RichardWattCV  See this post of mine for why Dell uses RAID as the factory default for all of their systems. Yes, the process you described would work.