anarchosax I wasn't able to retrieve anything from the hard disk, which I presume is wd0.
Actually it's dk0. From your dmesg output:
[ 2.245912] wd0: GPT GUID: 20fedeae-8e96-4489-83b8-02f5556d2730
[ 2.245912] dk0 at wd0: "83c710bb-b1ff-43c3-b596-39825bf37ea1", 3890440192 blocks at 2048, type: ffs
[ 2.245912] dk1 at wd0: "6c04e87c-8f3c-4363-b484-af10e2e98af7", 16586895 blocks at 3890442240, type: swap
anarchosax I also tried ffs, lfs, as well as mount without specifying a partition type...
The native partition type is ffs, but, on NetBSD (esp. with UEFI partitions, you can omit the type: the system will infer the correct type from the GPT table--as seen above)
OK, I can tell you why your system won't boot, but, not how you got there:
On GPT partitioned disks, NetBSD creates wedges for each partition in the GPT: dk0, dk1, ... as seen above.
On MBR partitioned disks, NetBSD creates slices for each partition defined in the (NetBSD) disklabel: wd0a, wd0b, ... for ATA disks (SCSI disks would go: sdNa, etc. where N is a disk number.)
It looks like you have a GPT-partitioned disk, but, for a UEFI BIOS to boot from this disk, you need an additional ESP partition--an MS-DOS or VFAT formatted partition--containing the \EFI\BOOT\BOOTX64.EFI, or, \EFI|BOOT\BOOTIA32.EFI UEFI bootloader files. This doesn't exist and, I don't know why it wasn't created during your install.
Try the install again, taking care to:
Choose the correct USB image.
There are 2 install images: a BIOS-only image and a standard one. Use the latter.
Turn off any CSM (BIOS compatibility mode) in the UEFI BIOS to not confuse the installer.
Create a (U)EFI ESP partition for the bootloader files.
pin but doing this is a waste of your time. It's enough to zero out the first Mb of the disk. If you feel unsure, zero out the first two or three Mb.
For OP's disk, with only NetBSD to be installed I agree. Zeroing-out the 1st MB would suffice. But, there is a caveat: if you want to install NetBSD in some existing free space on an already partitioned disk, you may have to zero-out the 1st MB of that free space too:
I had bizarre issues with sysinst while trying to install a test NetBSD partition on some free space I had on my GPT disk. I had previously created (and deleted) other partitions in that same space and sysinst got horribly confused because it read some old data from that "free" space (I guess because it uses gpt -b <blocknr> for some of these operations).
I only got sysinst to work by wiping the "free" space--which is not easy because, well, free space doesn't have a wedge associated with it so you can't go dd if=/dev/zero of=/dev/rdkN bs=1m count=1. You first have to create a wedge, wipe it, then delete it. (The other alternative is to calculate the correct offset for a dd seek=NN, but, I get muddled with the difference between seek= and skip= and block numbers vs. sector nums. and other arcana...)
