There are special precautions to take when using ZFS under a hypervisor, like Proxmox. If not taken, data loss or even entire pool loss is possible.
First, please describe how you pass through the disks to TrueNAS.
Next, please try these 3 commands, (using SUDO if not using as root). Then post the output in CODE tags.
zpool status -L
zpool import -d /dev/sdc
zdb -l /dev/sdc
The first will show which disks are part of which pool. So we can confirm sdc is not in another pool.
The second will likely show nothing or simple error. You have indicated that importing by name did not work. But, its harmless to try. (Because it is possible you had a typo or upper / lower case issue with the pool name.)
A bit of clarity. ZFS uses pools of disk(s), which must be in a vDev of some sort, (even single disks are an 1 disk Mirror vDev, sort of).
So what I think you mean is that one of your single disk pools is offline.
For reference, here is some background of ZFS & power loss: