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: 
All models of the Atari ST1 had code in ROM to attempt to read and run a boot sector from the first sector an ACSI2-attached drive. The bootloader3 contained in that sector would4 load the hard disk driver from a known file5 in the hard drive's root directory6. This program would install the full hard disk driver and boot would proceed normally using the hard drive's root directory, `AUTO` folder, etc.

1 The very first version of the 520ST loaded TOS from floppy disk. I am unsure of how it behaved when presented with a hard-drive at boot time.

2 ACSI was very closely related to SCSI, [see here for details](https://temlib.org/AtariForumWiki/index.php/ACSI,_SCSI_and_IDE). All of the hard drives I used on the Atari ST/TT were natively SCSI with a small adapter (ICD Link 2, about 2×4×6 cm IIRC) placed inline between the ST's ACSI port and the SCSI cable. Later models such as the TT used SCSI natively and booted directly from their internal hard drive; the TT provided both ACSI and SCSI-2 ports on its back panel. The Falcon was a bit different; it used an internal IDE bus (PIO, no DMA) and an external (DMA-capable) SCSI-2 port (no ACSI port was provided).

3 The bootloader would be installed onto the hard drive using a stand-alone utility, typically named `HINSTALL.PRG`.

4 Based on a combination of my recollections and the [info on this page](https://temlib.org/AtariForumWiki/index.php/AHDI). AFAICR I haven't used a hard drive with any 520ST model only the 1040STf (early, TOS 1.0), 1040STe (TOS 1.62 or 2.06, forget which), and TT030 (TOS 3.x) models.

5 Originally this was `SHDRIVER.SYS` (for SH204, SH205, and Megafile) and I believe was specific to the drive's controller board. I used the third-party `ICDBOOT.SYS` (for a 40MB drive) and `SUPBOOT.SYS` (for a 720MB Supra drive) on my drives. I don't recall what the TT's internal drive used (the driver may have been built into the TT's version of TOS).

6 One could also install the hard disk driver by placing the driver (`AHDI.PRG`) in the `AUTO` folder of one's normal boot (floppy) disk. The `SHDRIVER.SYS` file is simply a renamed copy of this program.
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.