HDD not always detected in BIOS
As drives get older, they can take longer to get up to speed and become ready (available). This can then exceed the time it takes for the booting to look for the drive - so it doesn't find it.
The solution is often to make the initial part of the boot take longer, by disabling "Quick power on self test and/or making the system look for the floppy drive first by enabling "floppy seek".
Some BIOS even have a facility to put in a delay before the hard disk is checked.