Here are some possible courses of action re your drive:
You could remove the drive and rig a cable to temporarily install it as an external drive (setting the SCSI ID temporarily to a non-zero available number) on another Mac and then backup any files that you want to save. After the backup you could run repair or disk recovery software (Disk First Aid, Mac Tools, Norton Utilities, or similar) on your known good Mac against the external drive to look for issues. If there is any doubt you have enough free space you could free up some, defragment the disk while still on the known good Mac and then try it again back on the 840av. If still problems, see reformat/reinstall below.
You can leave it in place initially and try to externally boot from some utility software like Norton Utilities or Mac Tools or even a MacOS installation CD just to get to a point where you can look at the hard disk directory and verify first of all that there is plenty of free space (as in many, many multiples of your main memory size). Then running a disk repair operation (Disk First Aid) or disk recovery program is a good idea, along with an anti-virus scan. This gives quite a workout to reading the disk but requires the CPU and memory to be good, and I have some reservations about attempting a defrag at this point on the 840av. If all appears well it is likely but not certain that the CPU, memory and power supply are good, but disk write operation integrity is not known and importantly there is a big question of whether the driver, boot blocks and MacOS image are all good or corrupt. Nothing quite like a reformat writing zeroes or ones with read verify after write followed by a clean install to reassure at this point, so a backup might be good first on another Mac, previously discussed.
You could remove the drive and rig a cable to temporarily install it as an external drive (setting the SCSI ID temporarily to a non-zero available number) on another Mac and then backup any files that you want to save. After the backup you could run repair or disk recovery software (Disk First Aid, Mac Tools, Norton Utilities, or similar) on your known good Mac against the external drive to look for issues. If there is any doubt you have enough free space you could free up some, defragment the disk while still on the known good Mac and then try it again back on the 840av. If still problems, see reformat/reinstall below.
You can leave it in place initially and try to externally boot from some utility software like Norton Utilities or Mac Tools or even a MacOS installation CD just to get to a point where you can look at the hard disk directory and verify first of all that there is plenty of free space (as in many, many multiples of your main memory size). Then running a disk repair operation (Disk First Aid) or disk recovery program is a good idea, along with an anti-virus scan. This gives quite a workout to reading the disk but requires the CPU and memory to be good, and I have some reservations about attempting a defrag at this point on the 840av. If all appears well it is likely but not certain that the CPU, memory and power supply are good, but disk write operation integrity is not known and importantly there is a big question of whether the driver, boot blocks and MacOS image are all good or corrupt. Nothing quite like a reformat writing zeroes or ones with read verify after write followed by a clean install to reassure at this point, so a backup might be good first on another Mac, previously discussed.