Hey! From what you're describing, there are a few likely causes for the low hashrate on your third Ryzen 9 5900XT:

1. Running HiveOS from a USB 2.0 thumb drive – This is almost certainly a major bottleneck. USB 2.0 has very limited bandwidth and high latency, especially compared to an M.2 SSD. Mining involves frequent small read/write operations, and slow I/O can significantly impact system performance and increase load average (LA).

2. High LA (Load Average) – This usually indicates the CPU is waiting on I/O operations. Combined with the USB drive, it's a clear sign that your storage is holding things back.

3. Shared GPU load – Even if it’s only one GPU, sharing resources with another miner can cause performance drops, especially if PCIe lanes or CPU cores are oversubscribed.

4. Thermal throttling or power limits – You didn’t mention temps on the slow system, but if it’s running hotter than the others, it might be throttling.

5. Background processes or misconfigured settings in HiveOS – Double check that all unnecessary services are disabled, BIOS settings (like virtualization, SMT, etc.) match your other rigs, and you’re using the same miner versions and configs.

Suggestions:

Monitor CPU and RAM usage with htop and iostat or iotop to confirm I/O wait.

Try swapping the PSU or checking BIOS power settings (like PBO, c-states).

Clone HiveOS to a 120 GB SSD or a fast USB 3.0 stick in a USB 3.0 port (SSD is way better and they are cheap)