I just tried several WD My Passport Ultra that I have. My DX200 has no problems seeing them even without a powered hub: just USB-C OTG + original WD cable. It detected old 2TB and 3TB models with MBR partitioning, and it also detected a new
yellow My Passport 4TB GPT-partitioned drive.
It showed a problem with relatively new 3TB drive, GPT, but factory formatted, with 4KB clusters. That 4TB drive was formatted with 64KB clusters. All my currently in use WD HDDs are formatted with 64KB (NTFS) or 128KB (exFAT) clusters, for faster access. Also, it means, they have less than 2^32 clusters in total, which can be a limit for Android. I'll try to re-format that 3TB drive later to see.
Please note: I use WD My Passport Ultra drives for many years, with DX90, DX80, and now with DX200. But I never heard success stories with Seagate, or any other models. The problems somehow were related to drive's power management, but I don't really care because all my 9 WDs, from three different generations, work flawlessly
In brief: DX200 supports both MBR and GPT-formatted HDDs, vFAT/exFAT/NTFS, 2-3-4TB are confirmed. 64KB cluster size is recommended.