Unraid 7.3.2 VM on Proxmox: USB boot flash disappeared while running, WebGUI failed and shutdown hung with rc.6 I/O error

Hello,

I am looking for help investigating what appears to be my Unraid USB boot device dropping offline while Unraid was running as a VM on Proxmox. This setup had been stable for several months and no hardware or VM configuration changes had been made recently.

Environment

Unraid version: 7.3.2

Hypervisor: Proxmox VE 9.2.3

Proxmox kernel: 7.0.12-1-pve

VM ID: 100

VM name: Unraid

VM configuration visible in Proxmox:

- BIOS: SeaBIOS

- Machine: i440fx

- RAM: approximately 20.94 GiB

- CPU: 16 cores, type host

- Boot helper ISO: plopkexec.iso

- Network: VirtIO

- USB boot device passthrough:

usb0: host=0781:5567

- PCI passthrough devices:

0000:05:00

0000:00:1f.2

0000:02:00.0

The Unraid boot device is:

Manufacturer: SanDisk

Product: Cruzer Blade

Vendor/Product ID: 0781:5567

Capacity: approximately 15.4 GB / 14.3 GiB

The physical Proxmox host is connected to an APC UPS. The UPS is attached to the Proxmox host over USB and remained visible:

Vendor/Product ID: 051d:0002

Product: APC Back-UPS BX 1500BI-BR

Firmware: V586

There was no known power outage and the Proxmox host itself did not reboot.

Today, at approximately 02:49BRT I tried to SMB into my shared folder. Got an error. I then opened the Proxmox console for the Unraid VM. Console login as root still worked, so I executed: reboot. The console displayed:

Broadcast message from root@tower (tty1) (Wed Jul 15 02:49:34 2026):

The system is going down for reboot NOW!

INIT: Sending processes configured via /etc/inittab the TERM signal

/etc/rc.d/rc.6: line 22: /etc/rc.d/rc.6: Input/output error

INIT: no more processes left in this runlevel

The VM then remained stuck indefinitely. It did not reboot or power off. From the Proxmox host I attempted a graceful shutdown:

qm shutdown 100 --timeout 300

The result was:

VM quit/powerdown failed - got timeout

I then attempted:

qm stop 100

The result was:

trying to acquire lock...

can't lock file '/var/lock/qemu-server/lock-100.conf' - got timeout

At that point the guest appeared to be completely stuck in the shutdown process. I ultimately had to force-reset/stop the VM from Proxmox. I ran usb-devices on the Proxmox host. At that time it listed the USB root hubs and the APC UPS, but the SanDisk Cruzer Blade was not present. The relevant USB controllers on the host were:

Linux 7.0.12-1-pve xhci_hcd

Controller PCI address: 0000:00:14.0

I then ran:

dmesg -w

I physically removed and reinserted the SanDisk USB flash drive. Immediately after reinserting it, the Proxmox kernel logged:

[861490.173243] usb 3-2: new high-speed USB device number 10 using xhci_hcd

[861490.297590] usb 3-2: New USB device found, idVendor=0781, idProduct=5567, bcdDevice=1.00

[861490.297595] usb 3-2: New USB device strings: Mfr=1, Product=2, SerialNumber=3

[861490.297597] usb 3-2: Product: Cruzer Blade

[861490.297598] usb 3-2: Manufacturer: SanDisk

[861490.297600] usb 3-2: SerialNumber: 04014718102423223929

[861490.298338] usb-storage 3-2:1.0: USB Mass Storage device detected

[861490.298438] scsi host0: usb-storage 3-2:1.0

[861491.331787] scsi 0:0:0:0: Direct-Access SanDisk Cruzer Blade 1.00 PQ: 0 ANSI: 6

[861491.333593] sd 0:0:0:0: Attached scsi generic sg0 type 0

[861491.346252] sd 0:0:0:0: [sda] 30081024 512-byte logical blocks: (15.4 GB/14.3 GiB)

[861491.347232] sd 0:0:0:0: [sda] Write Protect is off

[861491.347235] sd 0:0:0:0: [sda] Mode Sense: 43 00 00 00

[861491.347535] sd 0:0:0:0: [sda] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA

[861491.373854] sda: sda1

[861491.373919] sd 0:0:0:0: [sda] Attached SCSI removable disk

After the physical unplug/replug, the SanDisk appeared normally again on the Proxmox host. Its Vendor/Product ID matched the existing VM passthrough configuration:

USB Device (usb0): host=0781:5567

I started the VM again and Unraid is currently booting. I expect an automatic parity check because the shutdown was not completed normally. (Which is currently running)

I have a few questions.

1. Does this evidence indicate that the USB boot flash device itself dropped offline or stopped responding at the Proxmox host level while Unraid was running?

2. Is the /etc/rc.d/rc.6 Input/output error consistent with the boot flash becoming unavailable during runtime?

3. Could the boot flash disappearing explain the WebGUI login failure even though the root console was still accessible?

4. Is there any known problem with Unraid 7.3.2 running under Proxmox VE 9.2.3 using QEMU USB passthrough by Vendor/Product ID?

5. Is this more likely to be:

- a failing SanDisk flash drive;

- a USB port/contact problem;

- an xHCI controller issue;

- a QEMU/libusb passthrough issue;

- or an Unraid issue?

6. Would mapping the device using its physical USB port, for example host=3-2, provide any advantage over host=0781:5567?

7. Should I replace the USB boot device immediately, or first test the filesystem and move it to another USB 2.0 port?

8. Are there any specific Proxmox host logs, Unraid diagnostics or flash files that would help determine why the device dropped offline?

I will attach the complete Unraid diagnostics ZIP generated after this boot.

tower-diagnostics-20260715-0341.zip