EDIT: Solved. Used ImgUSB (recommended by Proxmox website). Rufus is apparently not compatible with the Proxmox image. Strange, since it seems to work fine with other bootable linux images.
Hello, I came across this thread while searching for a similar error. It may be that Alessandro 123 is mis-stating the issue here, and it may not actually be the technique he is using (which sounds like he's using a bootloader like GRUB on a USB stick with multiple OS images on it to select which to boot into).
I've created USB install media using the proxmox-ve_4.3-e7cdc165-2.iso image downloaded on Oct 7th, 2016 from this site, and using the program "Rufus" which I have found in the past to be a very reliable tool for creating bootable USB sticks. It should be noted that this is my very first experience with Proxmox, I've never installed or run it before - Rufus was used to create Ubuntu and CentOS installer or live media, for example.
Just like Alessandro 123, I am able to get the image to attempt to boot. I am even prompted with the Proxmox spash screen where I am asked if I'd like to Install PVE, Install PVE (Debug), Rescue Boot, or Mem Test. Here's where the problem begins (and what Alessandro 123 left out): After selecting either Install Proxmox VE or Install Proxmox VE (Debug), the system errors out during boot. After the "loading drivers:" line, I get the following errors:
modprobe: ERROR: could not insert 'ghash_clmulni_intel': Invalid argument
searching for cdrom
testing cdrom /dev/sr0
mount: can't umount /mnt: Invalid argument
testing again in 5 seconds
[ 4.963300] sd 2:0:0:0: [sda] No Caching mode page found
[ 4.963903] sd 3:0:0:0: [sdb] No Caching mode page found
[ 4.963905] sd 3:0:0:0: [sdb] Assuming drive cache: write through
[ 4.971196] sd 2:0:0:0: [sda] Assuming drive cache: write through
testing cdrom /dev/sr0
mount: can't umount /mnt: Invalid argument
testing cdrom /dev/sda
mount: can't umount /mnt: Invalid argument
testing cdrom /dev/sdb
mount: can't umount /mnt: Invalid argument
testing again in 5 seconds
(Note that this part repeats from the second "testing cdrom /dev/sr0" through "testing again in 5 seconds" 3 more times)
no cdrom found
unable to continue (type exit or CTRL-D to reboot)
/ #
Unfortunately at this point I am unable to do anything, but that may be my fault as the only keyboard I have available as a spare for this box (a Dell PowerEdge R710) is an old Apple keyboard that came with a Power Mac G4 (it's a USB keyboard and works most places, but under a barebones installer image I could see how there could be problems).
The system still goes into screen saver mode after some time (5 minutes?) and I can wake it up again by tapping the power button on the server (waking it via ACPI). So, I know the OS itself is "alive", it just doesn't seem to see any devices when booting. BIOS sees the DVD-ROM drive. This server does NOT currently have a RAID controller or any hard drives installed in it - only another USB stick which was to be the installation target (which I know is not a good idea, but this is only for quick tests and I don't care if this USB stick gets burned up). Is my lack of more durable storage devices causing a problem for the installer?
I haven't done anything fancy to my USB stick (no dual-boot, etc.). My problem, which sounds very similar to above, occurs after GRUB completes and the Proxmox VE installer image is attempting to boot.