Windows 10 dual boot UEFI USB HD Rocky Linux 9 solved

I fought with this for a while and hope it helps.
After trying WSL I knew I needed Rocky on metal. The 9 installer recognized only the USB bootable Rocky and internal SSD that Win OSs UEFI partition is on, and only that partition… After squishing Win partitions around and trying a myriad of approaches, still no joy. To be absolutely safe with my work PC I decided to install Rocky on a 1TB USB HD also unrecognized.
I downloaded and flashed a Rocky 8 live boot image to my USB stick, booted to it, and viola the USB-C external SSD showed up. I formatted it and installed Rocky 8 there to create UEFI and other partitions.
Then in Windows I wiped my USB stick and using Belana (not Rufus) I flashed Rocky 9 bootable and rebooted to that.
Then I was able to wipe Rocky 8 off my USB-C and it’s UEFI and replace with Rocky 9.
JOY!
Win 10 and Rocky 9 boot menu.
Also, I had to use my USB-C cable to charge my phone, and when the Rocky SSD was disconnected Windows boot worked as single-boot. I didn’t have to restore Win’s UEFI partition, and reboot after reconnecting Rocky USB SSD opened back to a Grub-looking bootlader that had OS choices! Seems stable.
I’m new to UEFI and anything Red Hat fork, so a bit out of my depth to have any explanations for why anything did or didn’t work, but it took a bit of trial and error/success time to make it happen. I didn’t expect Windows to boot OK after disconnecting the Rocky HD, but was pleasantly surprised.
I hope this info helps someone in a similar circumstance that must have Rocky 9, doesn’t want to risk corrupting or restoring a critical Windows internal HD or UEFI partition, and WSL is too weak to suffice.

A lot of whether you can boot from different devices easily depends on the motherboard firmware. Lenovo and AMI firmware is pretty accommodating, Acer not so much so.

Good to know. Fortunately in my case from the UEFI menu the MB had no issues recognizing UEFI partitions and bootable media. The major issue I was against was the Rocky 9 DVD ISO on a bootable USB wasn’t recognizing any writable disk hardware except the UEFI partition on my internal SSD, and the USB the bootable ISO was on. My Win C: partition is on a 1TB drive, but Rocky 9 was only returning the space of the UEFI partition which shares that same drive which would be a huge sacrifice if not paying attention.
Wondering if a Rocky 9 live image is planned, because using the Rocky 8 live image worked great, but is a convoluted way to go about a Rocky 9 install. Perhaps there are other better methods or better explanations?

Which writable disk hardware are you talking about, that’s “not” the internal SSD?

A 1TB USB SSD C-cable connected HD. I used a USB ‘stick’ flash-drive for the bootable install media in both cases, but UEFI boot partition for long-term OS install on the 1 TB C/USB drive, if that makes sense? In other words, an external hard drive for the OS and UEFI on the external hard drive, but for install media I used bootable images on a 500MB stick.
Rocky 9 DVD ISO, on the stick media, was only seeing the internal UEFI partition on the internal disk, as well as the stick USB it was booted from. In Windows MMC I squeezed out a partition then tried Parted (GParted) from WSL Debian, as I wasn’t able to run a ‘live’ Rocky 9 without the install prompts. The WSL route for creating ‘physical’ partitions was a bust, so between the Debian and Red Hat forks for anything live boot, I went with Rocky 8.
When I put the Rocky 8 Live image on the stick and booted, I was able to see all my disks and create the UEFI partition and OS partition on the external USB-C disk.
After re-flashing the stick with Rocky 9, I was able to install to the external SSD which it now recognized thanks to the Rocky 8 install and partitioning. After booting into Rocky 9, it was a matter of removing the Rocky 8 OS partition and the pointer to it from UEFI, and reclaiming disk space.
So, two external writable disks, a stick and external SSD. One for booting, one for integrating as an OS. Apologies if it wasn’t clear above.