I had a very freak electrical accident with my iMac 5K a few weeks ago.

One of my external drives (my backup drive) shorted out and would no longer mount.

I decided to buy an 3.5" enclosure as I figured it was just the power supply that died.

Now, when connected I get a "This drive could not be read…"

I've run numerous terminal commands and some third party software, and as far as I can tell, the data is all intact, but but it looks like the drive configuration and headers are messed up. I see errors such as:

"Check the harddisk size: HD jumper settings, BIOS detection..."

"…no primary or secondary GPT headers, can't recover…"

"…unrecognized file system (-69846)…"

I'm talking out of my butt here, but it seems like the drive headers(?) could be reset correctly and the partition would magically show up.

Here's the results of several scans. The drive in question is disk2, and disk2s1 appears to be the partition in question:

> diskutil list

/dev/disk2 (external, physical):
   #:         TYPE NAME                    SIZE          IDENTIFIER
   0:         FDisk_partition_scheme      *4.0 TB        disk2
   1:         0xEE                         500.1 GB      disk2s1
> sudo gpt show disk2

       start             size        index  contents
           0                1         PMBR
           1       7813971632  
> testdisk > analyze >

Disk /dev/rdisk2 - 4000 GB / 3725 GiB - 7813971633 sectors
Current partition structure:
     Partition                  Start        End    Size in sectors

Bad GPT partition, invalid signature.
Trying alternate GPT
Bad GPT partition, invalid signature.

Data Rescue returns what looks like a clean filesystem on its scan:

Data Rescue returns what looks like a clean filesystem on it's scan.

  • 2
    Take a look at DiskDrill. You can run their diagnostics for free and see if things are recoverable. Yes it costs money - things do. I've used this many times and I've had good success with it. "Free" doesn't necessarily mean better.
    – Allan
    Aug 25, 2020 at 15:45
  • There's also the possibility that either the old or [less likely] new enclosure is masking the sector size, which is an old Windows workaround for large partitions on FAT drives. There's an explanation here - klennet.com/notes/2018-04-14-usb-and-sector-size.aspx It might be worthwhile checking the make & model of your old enclosure to see if it was doing something like this [I doubt a new enclosure would.]
    – Tetsujin
    Aug 25, 2020 at 16:02
  • @allan — DiskDrill is good and I've used it before. The main problem with any of these restore programs is I'm going to end up with garbled files. The restore apps look for patterns they recognize as files and then you can copy them over from there. My entire partition looks like it's intact … I'd rather just repair the partition and have it like it was as opposed to dealing with the slag left over from a file recover, if possible.
    – Tomasch
    Aug 25, 2020 at 16:05
  • Recovery is about getting your data back, not restoring your drive. It doesn't garble anything, it recovers what was lost. If you want it the way it was, then you need a backup from which to restore. You say it looks intact...can you provide us with what you're seeing so we can work off of that?
    – Allan
    Aug 25, 2020 at 16:09
  • @Allan for sure, but restoring the drive in place, resulting in an intact drive, in it's original layout, would definitely be preferable to a more haphazard file restoration result. I've gone at this several different ways — what info from what sources would be best?
    – Tomasch
    Aug 25, 2020 at 16:25


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