@Maeson You need firstly to learn to difference between incompatible and wrong configured, then you can search between the threads. Most hdd drives work on a USB Loader, the most upcoming problem on hdd is simly they are wrong set up:
Wrong partitiontable, Filesystem format, wrong Allocation unit, oversized >2TB, wrong or no Y-Cable, multipartition with GPT (dont work), using wrong port. Or if games dont run, wrong named folder, ID, ISO/wbfs or corrupted images, wrong settings inside USBLGX.
Then they work.
On a USB Flashdrive is using the WBFS filesystem the last suggestion. If this dont work, the flashdrive is incompatible.
The main problem is inside the cIOS patch, the d2x has a USB modul "Mini EHCI" thats not official and gives transferspeed of USB 2.0. Its this simplyfied version of Intels own implementation of "EHCI" (wich is licened) and causes this incompatibility. This modul was implemented as USB 1.0 with very low tranferrates was still the standard.
The IOS 58 has the official USB 2.0 Support created by the USB consortium. So using IOS 58 or cIOS(base58) as Loader IOS may work as workaround if issues appear with drive recognition.
And next time i will give the thread to YOU if a Flashdrive problem comes up. I am sure you will troubleshoot it.
//EDIT: Dont miss understand, as iam taking this discusion the multiple thousand times. If the USB Flashdrive works flawless its nice, its what we all want. But OUR experience says NO, they are mostly the pain in your ass.
BTW, because of this incompatibility issues the Developer cyan of USB Loader GX asked once the Developer of the cIOS to remove this "Mini EHCI" Modul and implement the official USB 2.0 support. But never happened.