M1 Max Mac Studio: watchdog panics with an external USB4 NVMe startup disk

I'm trying to diagnose repeated freezes and kernel panics when running macOS from an external SSD. I would appreciate advice on how to identify the underlying failure, rather than assume that the SSD, enclosure, or a particular application is responsible.

Hardware and software:

- Mac Studio with M1 Max (Mac13,1).

- HAGiBiS USB4 NVMe enclosure, advertised as using the ASMedia ASM2464, connected directly to a rear port.

- Fanxiang S500Pro 2TB SSD, firmware GTdecda2, APFS, approximately 578 GB free.

- Both the internal and external installations report macOS 27.2, build 26B123. The internal installation boots successfully.

What happened:

I updated the internal installation first, then the external installation. After the external update, normal startup repeatedly ended with a black screen, a brief purple flash, and a restart. Safe Mode on the external installation worked. Similar external-startup problems have happened after previous updates, sometimes resolving after several restart attempts.

One startup panic reported:

IOKit Daemon (kernelmanagerd) stall[0], (60s): registry root held busy, kernelmanagerd has not checked in

The original connection negotiated USB 10Gbps. After replacing the cable with a 40Gbps-rated cable, the enclosure was detected in USB4 mode at 40Gbps and the SSD appeared via PCI-Express/NVMe. Normal external startup became possible. Third-party system extensions were also removed during troubleshooting, so several things changed around that time.

However, the Mac later froze and restarted during ordinary use, and some subsequent boots froze again. Four recent panic reports contain:

watchdog timeout: no checkins from watchdogd in 91 [or 93] seconds

Three reports captured a nezha-agent monitoring thread with APFS in its kernel backtrace. Another captured a WeApp thread; that report also contained a nezha-agent thread in an APFS/storage call. The repeated Nezha/APFS association is a clue, but I understand it does not establish causation. Nezha has now been uninstalled; stability after removal has not yet been established.

Other observations:

- The Logitech G HUB HID/audio and Xiaomi virtual-camera system extensions were removed. systemextensionsctl list now shows 0 extensions. SIP remains enabled.

- SMART reads show no critical warning, media errors, or accumulated high-temperature warning/critical time. SSD monitoring showed around 41-43 C, although this does not measure the enclosure's bridge-chip temperature or guarantee the temperature at the exact instant of failure.

- A temporary benchmark slowdown after the cable change later recovered to about 1,560 MB/s write and 2,675 MB/s read.

- I have the full panic reports and can retrieve persisted unified logs around the incidents. I have not confirmed an actual USB4/NVMe disconnect or reset before a panic.

What would be the most useful next diagnostic steps?

1. Which specific unified-log processes/subsystems/messages or panic/stackshot fields could distinguish a USB4/NVMe disconnect/reset from an APFS or software-related stall?

2. Would booting from the internal installation and testing the same external SSD as a data disk be a useful comparison? What results would help distinguish an enclosure/SSD problem from an external-startup problem?

3. Are there documented issues with this Mac/ASM2464 combination, and how should enclosure firmware or power-management compatibility be checked with the manufacturer?

I would prefer to collect useful evidence before erasing the drive, reinstalling macOS, or replacing hardware. Thank you.