@barkas: Check dmesg (and/or the full logs (showlog)), the reboot reason is logged, and a watchdog reboot after a hang has a specific flavor that's logged

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And, no, I'd stay away from 5.6.x, but that's just me

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The userstore is originally mounted in a fairly straightforward manner, (/mnt/base-us) but it's then proxied through fuse (/mnt/us) to handle USBMS somewhat gracefully, so it's potentially unpredictable ^^.