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I have a 4 TB hard drive that used to be inside a Seagate Backup Plus enclosure. The drive is Advanced Format so when I mount it internally or using another enclosure its partition is not readable. I'm trying to mount it on a mac and Disk Utility reports the partition is 500 GB. However it does see that the disk itself is 4 TB.

Is there anyway I can fix the partition table on the drive (or something) so that it's not advanced format anymore, without losing its data? Or do I have to completely reformat it for it to be usable?

edit: I'm running macOS 10.12.5. The drive was originally formatted using a GPT partition table with one 4 TB HFS+ partition. It is now seen as an MBR drive with one 512 GB partition of unknown format.

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There are some usb enclosures out there that emulate 4K drives for 512 bytes-per-sector discs. This means when the disc is taken out of the enclosure, sector addresses are not working anymore, as they should be multiplied by 8 to work again.

HFS+ on the other hand is sector-size-indifferent (much as for example ext2/3/4 is, but contrary to FAT or NTFS)

So it is only a question of rewriting the partition table.

In 4K-Mode (i.e. inside the USB enclosure), you had Sector 0: MBR protective partition Sector 1: GPT Header Sector 2: GPT Data which indicates partition starts at n and ends at m Sector n: Start of Partition Sector m: End of Partition

in 512bytes mode this is now: Sector 0: MBR protective partition Sector 8: GPT Header Sector 16: GPT Data which indicates partition starts at n (wrong) and ends at m (wrong) Sector n*8: Start of Partition Sector m*8: End of Partition

Size of the partition is m-n, which is of course also 8 times larger (8×500GB = 4TB)

Note down the values for n and m (to be found in sector 16 at the area colored, in my example 0x22 (start) and 0xF42 (end) here

Then use a partitioning tool of your choice (ideally something like gnu parted where you can enter direct sector addresses), create a new empty GPT partition table and declare a new HFS+-Partition (with Partition Type GUID 48465300-0000-11AA-AA11-00306543ECAC, which stands for HFS+) with the noted down start and end addresses, but multiplied by 8. Note that the addresses are of course little-endian hexadecimal addresses, so they have to be read as such and also be written as such.

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  • What software are you using in your screenshot that lets you read raw hex data from a drive?
    – Bri Bri
    Commented Aug 30, 2017 at 18:52
  • I used wxHexEditor, but that is under Linux. Under Windows I’d use HxD. Don’t know about macos, though.
    – Ro-ee
    Commented Aug 30, 2017 at 19:07
  • This did the trick! I ended up using GPT fdisk to both read the old partition start and end sectors (so no need to fiddle around with hex editors) and also to write the new partition table. The one caveat that took me a little while to figure out is that sector alignment value also needs to be multiplied by 8 in order for the new partitions to work.
    – Bri Bri
    Commented Aug 30, 2017 at 23:26
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You can't change the sector size on your drive. That is determined by the manufacturer when the drive is made. All modern operating systems will be able to read the advanced format drive. https://en.wikipedia.org/wiki/Advanced_Format

Your problem may not be with the Advanced Format, it could be other things like encryption, or compatibility of GPT / NTFS with your system.

You didn't specify any hardware or OS versions. You need to make sure whatever version of operating system you are using can properly mount and read advanced format drives and also GPT and NTFS. http://knowledge.seagate.com/ka030000000tlsfAAA

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  • I've definitely seen enclosures which emulate a different sector size than the disk actually is. (Absolutely no idea why, though.) And yes, their owners ended up having this same kind of problem...
    – grawity_u1686
    Commented Aug 30, 2017 at 17:05
  • I've edited my original post to include more information, but I'll repeat here: I'm running macOS 10.12.5. The drive was originally formatted using a GPT partition table with one 4 TB HFS+ partition. It is now seen as an MBR drive with one 512 GB partition of unknown format. diskutil reported 4K block size while it was in the enclosure. The drive was not encrypted. In theory the HFS+ filesystem is there, but the partition table is wrong so the system can't see it. That's why I'm wondering if it's possible for me to make it readable again without erasing it.
    – Bri Bri
    Commented Aug 30, 2017 at 17:30

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