Found the problem with the "home assembled" EHD
I reported yesterday the USB bridge in EHD I assembled was reporting 512 byte sectors instead of the drive's native 4K sectors. In addition to performance issues with non-sector aligned I/O operations, the smaller sector size produces 8x larger logical block addresses for the same size partitions. For this 3TB drive, this change caused the start of the 8th partition to have a LBA > 2^32. The MSDOS partition table does not support starting address beyond 32 bits, so the format on the hopper appeared to get stuck in loop when trying to write value into the partition table. Armed with this info I'm trying a few experiments. First, since the 7th partition started just below the 2^32 limit, I decided to make it bigger consuming the remainder of the space on the drive. After formatting and attaching the hopper, it didn't like and wanted to reformat. I'm in the process of partitioning the drive with a GPT rather than a MSDOS partition table. GPT has been supported in Linux a long time, so I'm hoping the hopper will support it as well. It will be a little while before I can try this. I'll report back my findings.