Please give us the output of NetBSD kernel boot stage.  And try to send
BREAK to see kernel can respond to enter DDB kernel debugger.

> At this point I decided to trie to read the SCSI disk on another machine.
> But the only one I have is a Linux PC running a 2.2 kernel.
> It recognizes the disk on startup, but says:
>
> sda: unknown partition table
>
> And if I try to mount e.g. /dev/sda1 it complains
>
> /dev/sda1 is not a valid block device

It's reasonable because [1] NetBSD/pmax disklabel is not the same as DOS,
[2] BSD FFS filesystem design is not compatible with Linux.

> So this got us to the second question:
> Do I have any chance to recover my data mounting the disk on Linux?

It's too early to determine disk is ruined.  At very least, the disk can
be recovered by another NetBSD, say, NetBSD/i386, if corrupted.

Toru Nishimura