You don't image it. You drag and drop ISOs into a directory on the SSD, and the USB device emulates a DVD ROM drive with the contents. You choose the ISO through a small screen and button on the device.
Can confirm, also have 2 IODD devices for this purpose, though I usually grab my (simplest) USB stick that has netboot.xyz flashed to it, and just boot off that, and select whatever netboot image I want and have it get everything from the interwebs. No need to ever update this workflow.
With the iODD 2531: It has modes: CD, HDD, and dual. CD provides emulation only. HDD and dual expose an FAT32 drive volume that can contains a magic _ISO directory to store .iso's. One limitation though is it doesn't support fragmented (discontinuous) files, and so those need to be minimally defragmented occasionally.
It also has an issue with remembering the last mounted .iso if its filename is beyond a certain length, in which case it will instead load a random (although always the same) .iso in the same folder.
I mainly had this issue with the default Windows install image names.
Fragmentation can be a bit annoying, especially when using exFAT, which doesn't appear to have defragmentation tools available. It can be avoided by never deleting files and instead reformatting every so often.
That being said, it's still a fantastic tool because all the images "just work" everywhere a class-compliant USB optical drive would.
No sense using exFAT because it's not as widely-supported. Don't have to reformat fat32 because File Allocation Tables are extremely simple. Move all files off, and then move them back serially, and presto, no more fragmentation.
FAT32 is not an option because Windows images these days are all over 4GiB. exFAT is very widely supported; the alternative would be NTFS, which doesn't work well (no write support) on macOS.
