Input/output error on Raspberry PI

[Frederic Fauberteau <triaxx%NetBSD.org@localhost>]

I run NetBSD 9.99.77 (RPI) on a Raspberry Pi ARM1176JZ-S r0p7 (ARM11J V6ZK core)

In my daily insecurity report, I can see:
Checking setuid files and devices:
Setuid/device find errors:
find: fts_read: Input/output error

At the filesystem level, I forced a fsck that found the filesystem was clean.

At the disk level, I tried a dd if=/dev/sd0 of=rpi.img that finished without any error.

Do you have any idea to identify the source of the problem?