Mac – Creating exFAT partition (for PC & Mac) alongside bootable HFS+ partition

I tried various other search terms and found that this question is actually a duplicate of this one: OS X 10.6 Snow Leopard no longer mounting an external USB drive.

Here's what I did to fix:

1. Download and install GPT fdisk

2. Run gdisk and select problem drive:

   ~% sudo gdisk
   GPT fdisk (gdisk) version 0.8.1
   
   Type device filename, or press <Enter> to exit: /dev/disk3
   Partition table scan:
     MBR: protective
     BSD: not present
     APM: not present
     GPT: present
   
   Found valid GPT with protective MBR; using GPT.
   

3. Look at the partitions using the p command:

   Command (? for help): p
   Disk /dev/disk3: 1953525168 sectors, 931.5 GiB
   Logical sector size: 512 bytes
   Disk identifier (GUID): ABFA9105-73F4-4627-9890-9DECC55E86AC
   Partition table holds up to 128 entries
   First usable sector is 34, last usable sector is 1953525134
   Partitions will be aligned on 8-sector boundaries
   Total free space is 3389 sectors (1.7 MiB)
   
   Number  Start (sector)    End (sector)  Size       Code  Name
      1              40          409639   200.0 MiB   EF00  EFI System Partition
      2          409640      1367597143   651.9 GiB   0700  Time Machine Backups
      3      1367599104      1953523711   279.4 GiB   0700  Basic data partition
   

   In this case, it's partition 2 that's causing the problem. And the problem is that its type code is 0700, which means something other than HFS.

4. Use the t command to set the partition type to af00 - Apple HFS/HFS+.

   Command (? for help): t
   Partition number (1-3): 2
   Current type is 'Microsoft basic data'
   Hex code or GUID (L to show codes, Enter = af00): 
   Changed type of partition to 'Apple HFS/HFS+'
   

5. Use the w command to write the stuff back to disk and quit.

   Command (? for help): w
   
   Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
   PARTITIONS!!
   
   Do you want to proceed? (Y/N): y
   OK; writing new GUID partition table (GPT).
   Warning: The kernel may continue to use old or deleted partitions.
   You should reboot or remove the drive.
   The operation has completed successfully.
   

The HFS+ partition, and all of its contents, then became visible again to OS X.

Windows XP 32-bit cannot access GPT-formatted drives to my knowledge, thus to be able to use it with a Windows XP computer, you would reformat it in MBR. This can be done in Mac OS with Disk Utility (Under the "partition" tab), or from Disk Management in XP (I believe called "initialize"), and will destroy all data on the drive.

If you cannot reformat the drive, some products claim to allow mounting of GPT-formatted drives in Windows XP, though due to their cost I can't test or vouch for any of them.

From Microsoft:

GPT disks are supported only by Windows XP 64-Bit Edition. You cannot move GPT disks to computers running the 32-bit versions of Windows XP.