Question: What percentage of PRAM reset rebooting works?
Question: One partition or more than that?
Question: Which version of OSX?
There is still something fishy in your OS, or the HD is getting cranky.
Permissions may have been improperly altered during the OSX install, possibly combined with the PRAM being corrupted.
To check for and/or cure this problem, boot from the OSX Install CD, and run Disk Utility. In Disk Utility, Select the hard disk drive in the drive list on the left side of the Disk Utility window. Then click on the Repair DIsk Permissions button near the bottom just left of center.
Verify that you do (or do not) have a S.M.A.R.T. ATA drive (System Profiler/Hardware/ATA/ATA Bus).
This next part will require some reboots, I am assuming you can do it with the PRAM zap.
If you have a SMART ATA drive, and you are running OSX 10.5, then install SMARTReporter, and see what it tells you. It should be able to detect any type of intermittent disk failure.
OK, on to booting in Verbose mode so we can learn more about the failing boot process.
To boot up in Verbose mode: Reboot and hold down both <control> and V. Write down the last few lines of the humungus text output, and tell me about it here by editing your question, or by adding a comment to your question or to my answer.
If I can not see anything obvious at the end of the boot log output, you may want to zap the PRAM, and then follow that immediately with a clean install of OSX (yet again). Bad PRAM settings might be able to negatively impact the install. If you did zap the PRAM right before the last clean install (erased the whole drive, then installed clean OS)
If you had two partitions on your hard disk, you could set up another OSX install on the second partition. This allows a perfect backup/restore of the main boot partition, and sometimes comes in handy for troubleshooting all kinds of problems. The System Preference Startup DIsk allows you to choose which partition to boot from, and you can run it from the OSX install CD when your machine will not boot.
If you are going to do a clean install, you might want to try splitting the drive into two partitions first. Boot the CD, partition the HD to have two partitions, one about 10 GB for OSX only, and another for the remaining space.
Once this is done, you may want to do what is called rehoming, setting a location on your boot volume where the large volume will 'appear' to be. Then you can set up your system just like it was, only using two separate partitions.
WARNING - OSX must NEVER run out of disk space for the OS. This is very bad, and will leave you with a situation like you have here, with data loss, and having to reinstall OSX. You must keep track of the free space on the OSX drive. You probably want a free gigabyte for swapfiles.
An external FireWire HD, set up to boot the same version of OSX would be handy for troubleshooting purposes, as reMacberlin suggested. That would allow us to do some of the experiments we could do with the second partition...
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Thank you all for the helpful info. I will try your suggestions. The point about making sure to not run out of room on the HD might be the answer as the teacher who uses this has the HD almost full.