Seagate 500GB External HD won't mount-help!

Question

My external Seagate FreeAgent 500 GB hard drive changed its partition type to nothing. . .
here is what it looks like in Terminal:
imac-users-imac:~ imacuser$ diskutil info /dev/disk4
   Device Node:        /dev/disk4
   Device Identifier:  disk4
   Mount Point:       
   Volume Name:       
   Partition Type:    
   Bootable:           Not bootable
   Media Type:         Generic
   Protocol:           USB
   SMART Status:       Not Supported
   Total Size:         465.8 GB
   Free Space:         0.0 B
   Read Only:          No
   Ejectable:          Yes
   OS 9 Drivers:       No
   Low Level Format:   Not Supported
By contrast, one of the internal drives looks like this:
imac-users-imac:~ imacuser$ diskutil info /dev/disk2
   Device Node:        /dev/disk2
   Device Identifier:  disk2
   Mount Point:       
   Volume Name:       
   Partition Type:     Apple_partition_scheme
   Bootable:           Not bootable
   Media Type:         Generic
   Protocol:           ATA
   SMART Status:       Verified
   Total Size:         25.5 GB
   Free Space:         0.0 B
   Read Only:          No
   Ejectable:          No
   OS 9 Drivers:       Yes
   Low Level Format:   Not Supported
Any help is appreciated. I know I should have been backing up this drive.
I have over 10 years' worth of freelance work, plus 100s of reference photos,
websites, resumes, portfolios on it. I have been exploring testdisk, photorec,
Data rescue, Disk Utility (which sees the drive, and gives me this message:
Verifying volume “disk4s2”
Error: The underlying task reported failure on exit
1 non HFS volume checked
  Volume needs repair
The thing is, I have searched for individual files, and I can see the paths to them. However, I cannot mount them or send them
to another drive. I also receive this message in Terminal:
Error (-9958) encountered attempting to verify/repair disk4
Message was edited by: Michael Brugh, spelling correction

Answer 1

Your filesystem appears to be damaged beyond what Apple's Disk Utility is willing to repair.
Frankly it's insane to have one copy of 10 years worth of files you think are important, with that out of the way.
If you really need the data and want it back with the most likely chance of success of reading it all stop using it right now and contact a drive recovery company. They have the knowledge & expertise that could save data from a disk that may have a very limited amount of life left - sometimes falling disks will stop reading as physical damage gets worse. It will be expensive, but it's the best option they should have clean rooms for disassembling & checking it is operational. They may also know what different noises may mean - any abnormal sounds are clues to what the disk is doing (or not doing).
Another thing to consider – the external case or USB port, cable or power supply could be failing. Removing the disk & installing into another case may be a possible way to see the disks state. The age of the disk will dictate the type of case you need (SATA or ATA). Also see if another Mac mounts it - the OS could be failing not the disk.
These are the risks that your own attempts to recover or repair.
Causing more damage to the HD until it becomes totally unreadable, running several recovery or repair tools may simply cause more damage.
A physical failure can simply become fatal to the ability to read any data.
Repair tools can cause files to be lost or can recover many duplicates of data without the file names or folder structure - it can make sorting them impossible or very time consuming.
Risk damaging or altering all other connected disks … tools like photorec & testdisk are not forgiving of typo's you can easily be working with the wrong disk. Remove ALL the disks that are cannot afford to be lost (it sounds extreme, but you are already in a bad situation don't make it worse). Device names are confusing, verify twice before doing anything.
You need another disk to recover data onto this needs to be larger than the 500GB drive - you will get duplicates & may need to 'image' the entire drive before attempting repairs on the image (to limit more damage to a mechanical issue). Several disks may actually be required. ddrescue is a free tool to image failing disks.
Time is another important factor, recovery or repair can be very a slow process & should also not be rushed, spend time reading manuals & guides before using tools that are new to you. Also read other sites that discuss the tools.
It's unclear what the actual issue here is & what options you would like to take. You could just try every utility you can find, but that will cause the disk to die if it is having mechanical issues. You need to be methodical & log what info you have.
Are you really on a G4 Power Mac with 10.4.11? You will need older versions of tools like Disk Warrior to handle the disk format correctly, otherwise they may fail or cause other damage.