I've been struggling to re-install 12.04 on my desktop all day. I'd previously used the upgrade path from the Update Manager, so I hadn't experienced the problem before. Try as I might, I couldn't get the PC to boot from the install CD. Changing the CMOS settings and boot order had no effect and I was plagued by a Drive not Available error message.
I tried resetting the BIOS - no effect.
I checked that the drive cables were properly seated - they were
I even checked that the drive was working - it was!
In an act of sheer desperation, I tried the install disc for 11.04 LTS and it installed flawlessly! I'm now in the process of upgrading.
I've checked the Ubuntu downloads pages for clues and found nothing that matched my circumstances, but I suspect that it's a hardware problem and the BIOS simply doesn't support the bootstrap. Anyway, if you're experiencing similar problems, it may be worth downgrading first.
Sources & References:
- Ubuntu Documentation - Boot From CD