Ubuntu Server 14.04.2 install error: can't umount /media [closed]

I'm trying to install Ubuntu Server 14.04.2 on an old Athlon 64, but as soon as I get past the initial menu, choosing to install Ubuntu Server, I get some console output and everything gets stuck on:

mount: mounting /dev/sda on /media failed: Invalid argument
umount: can't umount /media: Invalid argument
mount: mounting /dev/sdb on /media failed: Invalid argument
umount: can't umount /media: Invalid argument

I used the exact same USB stick to install on another box. I couldn't find anything else about this error other than this topic, where one user claims it eventually works, but in my case I left it sitting there for almost an hour and nothing.

I found some brutal but effective method :-) When I reach that error I simply pulled out the USB stick wait for about 10 seconds and push it in again. Waiting for another 10 seconds, enough to be mounted. After that, I choose to retry the CDROM search and... voila, the installation goes on. Once again, it is brutal and can damage your USB stick, but it worked for me.

So, after some time the installation resumed, but it fails on the next step, trying to mount a cdrom, which doesn't exist since I'm booting from USB. I skipped that step and manually created a link from /media at /cdrom and it works.