Last night I needed to run Seagate's diagnostic utility,

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These may be found on the Dell Drivers and Downloads page. Drivers for non-Dell systems must be found elsewhere. Windows 7 Official.iso Files (Digital River) Note these.iso files may be used with a Dell/non-Dell OEM product Key but phone activation is required (see note at end). Download or install either 7-Zip (free) or WinRAR (not free). Extract the downloaded.iso file. Rename the SeaTools.ima to SeaTools.img. Burn the SeaTools.img file using Win32 Disk Imager. Download and double click to execute it. Select teh USB device, and browse for the renamed.img image. Provides Direct customers with B2B Self Service tools such as Pricing, Programs, Ordering, Returns and Billing. Seagate Supplier Portal. Provides Suppliers with self-service tools targeted to the needs of their business. Lyve Management Portal. Register, access and manage Lyve Data Services subscriptions and projects. AllBootDisks ISO Image Downloads These are the ISO boot disk images available from AllBootDisks. Download the ISO image you need, and if you need assistance creating a bootable CD from this image, visit the how-to page. 1- Download SeaTools for DOS!! 2- Use 7-Zip to extract the ISO image. 3- Go to the extracted folder and rename the file SeaTools.ima to SeaTools.img (optionla) 4- Download Win32 IW. 5- Insert FAT32 USB and run Win32DiskImage, Select Seatools.img and burn your USB. 6- Restart your Computer.

SeaTools, against an old Maxtor drive so I could verify it as viable for installing a new NAS onto (more on that nightmare of a project in a different post). Seagate worked against me on this, instead of with me.
They have a version that runs in Windows, but this machine doesn't have Windows. It has FreeNAS (based on FreeBSD) on another drive, but I preferred not to have that drive hooked up during the testing.
They have a bootable CD image in the form of an ISO, but this machine doesn't have a CD-ROM. They claim it runs FreeDOS, so I naively figured I could use UNetBootin to throw it on a USB drive and boot from that. No dice.
To make a long story short, I managed to get it working with a combination of syslinux, memdisk, and vanilla FreeDOS. But I had a lot of trouble finding the information I need online. Most of what I found didn't apply to me (search for 'boot usb iso linux' and all you get is how to install Linux from USB, not how to boot a custom ISO from USB and create it from Linux), and some of it was misleading (e.g. 'you must format it with FAT, erasing the boot sector, but not with mkdosfs' -- wrong).
So here's what I did, and hopefully someone else will find this useful:

1. Download UNetBootin, use it to install FreeDOS onto the USB drive straight from the GUI
2. Install syslinux onto the USB drive
3. Copy the memdisk bootloader onto the USB drive
4. Download the SeaTools ISO and copy it onto the USB drive
5. Create a file called syslinux.cfg with the contents below, and copy it onto the USB drive, replacing the one that UNetBootin put there

DEFAULT SeaTools
LABEL SeaTools
LINUX memdisk
INITRD SeaToolsDOS222ALL.576.ISO
APPEND iso

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This gave me a bootable USB drive that went straight into Seagate's custom-boot environment. I reckon I can use this same trick to boot any DOS-based utility like this.
Here's the command transcript:

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