Seconded on Cockpit project w File Sharing.

Probably not best practice, but it’s possible to install it on the PVE host itself since its ZFS manager and Identity manager plugins and other features fills some gaps in what Proxmox doesn’t do (or would have to drop to CLI to do).

Also recommend RClone in a systemd can take care of various file movements, syncs and backup tasks you may need against the host, vdumps or SMB file shares.

Suggest the typical hardware device troubleshooting. watch/tail your dmesg -w or kernel log as you add the extra drive. It’s curious that the system itself doesn’t crash, but from your description it still sounds like a power starvation concern or possibly high temperatures if this device is under heavy load.