I saw really lot of pages, threads about it, and can say only that most of it is far from good and useful. Errors, shallowness and often in some commercial purpose.
Now we have diverse SD card adapters, and probably most sold is ACSI2STM - in diverse variants. It changed since first releases, and now pushes so called GEMdrive mode .
Let see what is on Github about : How hard disks work on the Atari ST (https://github.com/retro16/acsi2stm/blob/stable/doc/tutorial.md)
Quote: "Remember that using ACSI drivers is discouraged, you should probably use GemDrive unless you have very specific needs or want an authentic user experience (read: having a hard time struggling against old buggy software)."
This is so idiotic, likely just in interest to force using GemDrive (so buy ACSI2STM and not some other adapter) . Old SW is not buggy at all. Sure, there are some bugs in some, but generally most of it is pretty decent and well tested. Unless thinking that it is bug that old SW does not support some new hard disk filesystems and like :-) Sure, it should support LFN, FAT32, 2 GB partitions (in era when usual hard disk sizes were about 100 MB) ...
Then there writes that it (ACSI port) has no real name - hahaha .
"Mega STE internal hard drive
Some models have internal hard drives, however it seems that only the Mega STE has an internal ACSI drive, sharing the DMA port. By default it is set on the device id 0, but this can be changed by internal DIP switches."
Stacy has it too. And it is not ACSI drive, it is SCSI drive + ACSI-SCSI adapter. Why people with so low Atari ST experience writes about this things at all ? Complete bad approach - but hey, they are sooo smart, so know it better than those who used such machines :)
AHDI driver is not full of bugs. And limitations are actually in TOS self. You can read about it on my site: Atari ST family and mass storage (https://atari.8bitchip.info/ASTfamMS.html)
TOS limitations can be overridden in different ways. Surely most useful and easy to implement is TOS/DOS compatible partitioning and hard disk driver. It does not need any HW changes, just SW - partitioner and hard disk driver - easy to install. And can do it even without floppy or Gotek.
The real question here is : with what SW most of nowadays Atari ST family users use them ? I'm pretty sure that it is gaming - playing good old games in first place. As lot of people plays even older, 8-bit games.
Now even can buy new version of Atari 2600 (with cartridge port for games on ROM cartridges) .
Sure, there is lot of quality productivity SW, but using them now, when we have several thousands time faster computers and much more advanced SW ? Now simple Internet browsing needs min 2-4 GB RAM .
So, my activity is about making good old games working from modern storage - what is now really cheap considering capacity. But making SW written for floppy run, with copy protections in looot of ways to work from mass storage (now SD cards, CF cards) is pretty hard. Even if game has hard disk install option (program what does it) it may not work good - some have install only to subdir in root of C: - not good for thousands of games on mass storage. So, it needs corrections too. Plus, and I saw many such cases - there is low level hard disk access in code - what means not access to some files, DIRs, by it's name, but to physical sectors . And that's what will fails with GemDrive . I wrote about it to man behind GemDrive, and just got reply in insulting style - he is smart, I'm nobody. Sure, after doing over 2500 games I know nothing ! This is real problem in 'Atari community' - some just push own ideas, limited knowledge, and do not listen to other's experience, knowledge (not so limited).
All in all, please do not believe everything what can see online. I did not say anything new, of course.
For running good old SW, games we don't need new TOS (like EmuTOS), new hard disk systems. Best way is compatible TOS version - so regular TOS 1.00 - 4.04 or my improved TOS versions, which are made to be compatible as much is possible with original TOS versions. And of course hard disk driver what is good with old SW and TOS/DOS compatible partitions. Latest mean easy data transfers with modern computers - Windows, Linux, MacOS . And lot of things can be done on them way faster than with old Ataris .
And what can see on ACSI2STM doc (https://github.com/retro16/acsi2stm/blob/stable/doc/compatibility.md)
Well, they even don't know that Falcon has no ACSI port.
Works with all SD cards except SDUC (was not tested yet). Well, my experience is that very high speed newer SD cards may have problems, especially by writing on them.
"In GemDrive mode, the SD card must be formatted either in FAT16, FAT32 or ExFAT format, with a single partition (the standard format for SD cards). The partition size is limited to 2TB."
So, only 1 partition. How it will look with some 10000 DIRs ? And surely will work slower than smaller size partitions. Yes, it is basically same is in Steem and Hatari - where 1 DIR on host is assigned as Atari drive.
TOS supports up to 14 partitions C-P . And more partitions means not lower speed, actually contrary - less data on 1 partition means faster work.
They say that few games with no copy protection will work. Yeah, considering originals. What about hard disk adaptations, patches ? As usual they have no clue what all is made.
"Software relying on bugs
These programs use the normal GEMDOS interface to access disk drives but rely on weird patterns or buggy TOS error codes to work properly."
And same idiocy as on page about how hard disks work on Atari (first post in this thread).
Buggy TOS error codes - huh ! They are not buggy. May be different than some later widely used ones - it is really so stoooopid to expect from SW done 40 years ago to be by some later standards .
"Software doing BIOS access
These programs access low level disks using BIOS or XBIOS interfaces.
Programs usually working like this:
Most disk utilities
Maybe some very weirdly programmed games
"
It is not weird to use (X)BIOS call to determine is it running from floppy or hard disk. I saw it many times, and is usually by games with hard disk install option, or declared that work from hard disk.
And they mentioning my early, freeware ACSI driver from 2008. I did lot of it from then.
What about extra functions like Virtual Floppy, automatic Desktop drive icons creation ? By them, it is also bug of TOS that only icon for C is created ... Is it bug that TOS is in limited size ROM ?
Lot of it is done with compromises because HW prices were pretty much higher then.
And really, why such huge disrespect toward Atari ST designers, OS programmers ? Sure, there were some smaller errors, maybe few bugs which show in rare cases. Just one example: it crashes when in drive Window in Desktop some file is of size 10 MB or more (TOS 1.xx) . Well, I did not know about it when I used my Atari at 1992 with 40-160 MB hard disks, simply because were no so large files. I saw it first time somewhere at 2008, when used Steem with it's hard disk system . And. btw I fixed problem in my iTOS .