I'm not sure if it's appropriate to bump this now after such a long period. ArcaOS 5.1.0 has been out for a while and I just had some time actually giving it a try. Originally I planned to join the discussion on OS2World but for some reasons my credentials no longer work on its forum section anymore... only on the main site. It's been quite a while since I logged in there anyway...
The best part is formal UEFI support starting with 5.1.0. From my experience with the installation on a 512GB NVMe drive with GPT and with an existing Linux install, there were much fewer hassle compared to how it was on 5.0.x, and I was able to start and finish installation with no serious obstacles. My test on this part is limited as I still have several unanswered questions:
1. I need to further test this on a system that has several disks, being larger than 512GB and with both MBR and GPT ones, to see if the LVM wrapper that made GPT work would also address existing partitioning hassles I had with MBR disks on 5.0.x.
2. Does ANCS get involved in case of booting with legacy BIOS/CSM? For now it seems ANCS is doing a better job on UEFI, which I'll explain it later.
3. Mounting ArcaOS' JFS partition on a different OS namely Linux remain a question that I don't know where I could find clues for. I never had any success trying to mount the system JFS on my Arch-based Linux distros, so I can't really do any external maintenance in case something goes wrong.
4. Will probably need to retest the partitioning setup to see if I can somehow assign a previously allocated partition for ArcaOS install. Back then I wasn't sure and I ended up deleting that allocated space and recreated it as ArcaOS Type 1 which it expects. Although no harm done to adjacent partitions, the partition numbers were a bit messed up (as it does not try to reorder the partition numbers). Not sure if it will not break if I manually reorder the partition numbers from Linux.
Other hardware support notes:
1. Support for network adapters is still mostly limited to Gigabit (1GBe) ones. At least with MMRE (Realtek), 2.5GBe adapters do not appear to work, so on my test machine I have to switch to a 1GBe port to be able to use internet there. This may be a problem with very recent high-end systems that do not have 1GBe anymore, with 2.5GBe becoming more and more common there.
2. Some USB keyboards (namely Corsair ones) still cannot work. I still need to use another keyboard for the purpose. It may take a while before this issue finally gets resolved.
3. You can find actively updated UNIAUD drivers here which one could use in case audio does not work out-of-box. Check the latest posts there for most recent developments as well as possible arguments needed for specific system configurations. In my case, I had to manually specify the card ID (/A:1) to be able to actually get sound, as the first card on my test system happened to be HDMI instead of onboard audio codec.
4. USB Mass Storage works but I haven't tested it intensively, especially when it comes to hotplug (without safely remove).
5. Some in OS2World suggest disabling HyperThreading when using ArcaOS but in my case I don't really see any major issue with it kept on yet. Your mileage may vary.
Software-related:
1. With the help of ANCS, on my UEFI test system Win/OS2 is finally working (yay!). Previously on 5.0.x trying to start Win/OS2 would very likely lead to a freeze. This is a major breakthrough for enabling using ArcaOS for retro purposes, though there's still a way to go to be able to use sound there. Apparently there's something called DTA/DTAudio for supporting audio in DOS or Win/OS2 sessions but it's not available yet...
2. ArcaOS' Firefox 45 is a bit too dated for certain websites. Namely I had difficulty browsing GitHub with many things appearing broken (including releases page). Does anyone know of a browser good enough for most websites for ArcaOS?
3. Is there a better file manager for ArcaOS that feels more friendly for Linux/Windows users? The default desktop seems to follow a rather different philosophy that I find it a bit odd, though I admit I'm very noob when it comes to OS/2 in general, so I'm not in a position to say whether or not certain behaviors are indeed bugs.
- Moving/copying files are only limited to windows I already opened. As such, if I want to copy a file to somewhere I have to open that folder before I open the copy dialog.
- It seems it is actually possible for files/folders to have newlines in their names. Pressing ENTER when creating a new folder, for example, would not finish setting the name, but rather put the newline into its name.
- From what I could remember, it is actually possible to copy stuffs to and from unintended locations such as "Drives" folder...
4. While ArcaOS has the OS/2 installer program which I think it's for installing more features, it doesn't appear to work as desired and may lead to side effects. When I tried to install fonts for additional languages through the installer, I did not see any noticeable differences (probably the files were already installed), and it seems to have caused some side effects in the system config files, which rendered the system unbootable (it reported an error with COM.SYS and froze there). Probably it was due to my leaving the already checked option "Symmetric Multiprocessing" as-is (since I wasn't really sure).
EDIT: Some update as I re-attempted to install ArcaOS on that same system, though using a larger NVMe drive.
1. It's trivial to just change partition type to ArcaOS Type 1 via "Set name and letter". For GPT, you can assign just any drive letter to a partition the same way as how it's done on 5.0.x with MBR. There's no need to recreate any partition. I didn't test partition reordering, but I don't think it matters much considering
2. It's recommended that you name your partitions on GPT disks, as this is the one used by ArcaOS when using "Set name and letter". Note that partition name is specific to GPT and is different from the filesystem label which we already know about.
3. I haven't tested multiple disk scenarios involving mixed MBR/GPT disks. ArcaOS 5.1 is better suited for more recent UEFI-based hardware and it's already making good process to unleash retro potential on such systems.
4. I noticed that if a JFS partition exists in the system, Calamares (Linux installer used by many distros) would get stuck at partition module. I've filed a bug to Calamares regarding this discovery, as this means installing ArcaOS would prevent further install/reinstall of such a Linux distro that utilizes Calamares.
5. As I was testing reinstalling ArcaOS on the larger SSD, I noticed a strange phenomenon that the first boot after installer finished the first phase would hang. Rebooting the system (CTRL-ALT-DEL possible at this point) would allow it to continue to the second phase, but at first the install was kind of broken, leaving a bunch of RecoveredObjects as well as some trace-related stuffs, and without taskbar, but when I tried installing it again the install finished fine with everything in the right place, even though I also had the initial hang after first phase. For now I don't intend to mess with the system too much as ArcaOS still feels delicate for someone who's not experienced with OS/2, although it's now much easier to reinstall the system than before.
LATE EDIT (2024-01-14): I've struck through a few lines, as I got some time testing it on an old enough, BIOS/MBR based system. Sadly most if not all BIOS/MBR limitations present on 5.0.x still apply to 5.1.0 when using it on such systems.
- Disk size limitations (~512G for MBR), as well as the usual LVM quirks still apply. So you still need DFSee for MBR disks while you can use any partitioning tool for GPT ones.
- ANCS most likely won't be involved when using BIOS/MBR so most likely one won't notice too much difference compared to 5.0.x.
Additionally, even with GPT support ArcaOS still cannot work when the system has disks larger than 2TB. You MUST hide such disks via arguments of the respective drivers or the system will crash (trap) during boot process.