I was going to post this as a reply to the STM32F7 discussion -- itself a reference to the "Nine-Dollar Computer" discussion -- but I realized I should probably start a new thread. Lately I've been working with the Texas Instruments Tiva-C (ARM Cortex-M4) Launchpads, the $13 EK-TM4C123GXL and more recently the $20 EK-TM4C129XL.

Neither of these is going to be a Linux computer; they are aimed squarely at the embedded microcontroller market. So, lots of I/O, including USB and (on the '129) Ethernet, but no video input or output, no audio output, and no on-board SD card interface (though it can easily be added externally using one of the SPI ports). Both take standard Launchpad "boosterpacks" (add-on boards). What I particularly like about the '129 board is that with the addition of right-angle headers, it can plug directly into a solderless breadboard, with nearly all the I/O pins available. (This shows how it's done, though that's not the Tiva Launchpad in the photos.) The '129 chip also has an external memory bus, if 1 MB Flash and 256 KB RAM isn't sufficient for your needs.

No, I don't have a machine language monitor for it yet, but I am working on a Forth kernel.

I think they would make a good controller for an SD card or external devices if needed. Maybe a smart hard drive like the Commodore 1541 was?
I'm thinking it could free up the processor if needed and maybe it could write to the card or other drives if needed.
I've seen a scheme of what it takes to drive a hard drive and a lot of people don't know what is involved including me.

I'm guessing you don't literally mean to use the Commodore 1541 interface; I vaguely recall that was serial at 4800 baud -- very slow.

It's not particularly hard to interface to IDE (PATA) hard drives, since the controller is on the hard drive itself. (IDE = Integrated Drive Electronics) It's probably not worth trying to interface to any older hard drives (e.g. MFM or RLL), since those are long unavailable. Alas, I don't know enough about the modern SATA interface to be able to say if the Tiva-C can handle it.

I don't want to do any emulation of classic systems as I want to break from the past.
I do think that Commodore did some things that were smart; they recorded the program data twice on the datasette and used a checksum to make sure all of the data was there.

I would like to make a computer "Commodore like" but not copy the Commodore IP itself. I do think that I would do some things differently than the rest of the Computer companies.