Question: It has been my belief that one of the nice features of the original Atari ST is that it has support "out-of-the-box" for a hard disk drive connected via its built-in ACSI port- similar to the way you can connect an external SCSI drive to a Macintosh Plus and have it just work.

However, I see a lot of information online about hard disk driver software for the Atari ST. This usually comes with complicated caveats regarding TOS versions and HDD partitioning.

*Question*: Can a bare stock, original Atari 520ST, with TOS 1.0x ROM, boot from a connected hard drive, without any special boot floppy or 3rd party driver?

Assuming the answer is affirmative, what limitations are there with such a setup? And which 3rd party driver(s) or special partitioning software should be used to best overcome the limitations?

Answer: 
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> Question: Can a bare stock, original Atari 520ST, with TOS 1.0x ROM, boot from a connected hard drive, without any special boot floppy or 3rd party driver?
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Yes, as long as it's fitted with a proper partition/boot sector and hard disk driver (usually AHDI.SYS).

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> Assuming the answer is affirmative, what limitations are there with such a setup? And which 3rd party driver(s) or special partitioning software should be used to best overcome the limitations?
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There are none, just many ways to screw it when tinkering. Let's look how the Atari ST boot works:

* Look at the first Floppy (Drive A:) for an executable sector at Side 0, Track 1, Sector 1
	+ If one is found, go ahead and execute it
* If none is found check the AHCI drive (HD) for a a partition sector (again at logical sector 0
	+ If one is found, check if executable
	+ If executable, then do so
* If neither worked, bring up the default desktop, optional with a DESKTOP.INF if found on Drive A: (no matter if bootable or not)

Looks straight forward, so where do all these 'Drivers' come into play?

Well, they need to be loaded from disk, either Floppy or HD. On a properly initialized HD, the partition/boot sector contains a loader which loads AHDI.SYS (or whatever 3rd party to be used), which in turn hooks the RWABS vectors - Read or Write ABSolute (IIRC) - the OS vector to read a block from any device.

So in theory it would as well be fine if the driver is loaded from Floppy by some automated or manual process - except, the DOS layer only adds a C drive to the list of drives, if a proper partition/boot sector is found. Some drivers overcome this by 'informing' DOS independent of what it detected. That's also the way to install a new HD.

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Follow up question: *So why are there so many problems reported?*

Simple answer: PEOPLE.

Possible issues are:

* Having a bootable floppy (still very common back then)
* Deleting the driver
* Having the HD not ready when turning the ST on makes the DISK invisible
* Changing Partitions without correcting the bootable signature
* and several more

Personally I never had any issue with an ST Hard drive. Neither the original SH20 nor third party.
Yes you can boot any Atari/TT/Falcon from a hard disk you just need a driver there are a couple of commercial drivers still available with active support. Peter Putnik's driver he's still active on all the big atari forums and <https://www.hddriver.net/en/>

There are some limits on partition sizes -

TOS limitations:

TOS 1.00-1.02 : max partition size is 256 MB

TOS 1.04-4.02 : max partition size is 512 MB

TOS 4.04 - max p. size is 1 GB.

And I think there is a bug if memory serves me in TOS versions below 1.4 so if you want a hard disk on an STFM you really want 1.4 or on an STE or above 1.6 2.0x.