My neighbor brought over a 3TB external hard drive saying that after loaning it out to a Windows user, her Mac is asking her to initialize something whenever she plugs it in to her computer.I'm using Fedora, and I'm trying to recover any data off of the drive before I let her try anything on her computer, because I have a feeling she will lose the data if she let's her computer attempt to initialize the drive.I suspected the problem had something mangled with partition tables. Using fdisk I get the following output for the drive:Disk /dev/sdd: 2.7 TiB, 3000558944256 bytes, 732558336 sectorsUnits: sectors of 1 * 4096 = 4096 bytesSector size (logical/physical): 4096 bytes / 4096 bytesI/O size (minimum/optimal): 4096 bytes / 4096 bytesDisklabel type: gptDisk identifier: BAAE909E-8289-421C-A8D7-9DC750F0E342Device     Start       End   Sectors  Size Type/dev/sdd1      6     32773     32768  128M Microsoft reserved/dev/sdd2  33024 732558079 732525056  2.7T Microsoft basic dataUsign blkid, I get this:/dev/sdd: PTUUID=baae909e-8289-421c-a8d7-9dc750f0e342 PTTYPE=gptAnd using parted, I get this:Model: WD My Book 1230 (scsi)Disk /dev/sdd: 3001GBSector size (logical/physical): 4096B/4096BPartition Table: gptDisk Flags: Number  Start   End     Size    File system  Name                          Flags 1      24.6kB  134MB   134MB                Microsoft reserved partition  msftres 2      135MB   3001GB  3000GB               Basic data partition          msftdataI noticed immediately that it didn't have anything for the 'File system' column. How can I get this to mount in read-only at the least, even if it's just for me, so I can copy off the files she has on there?UPDATE 1Using file -sL /dev/sdd* produces:/dev/sdd:  ; partition 1 : ID=0xee, start-CHS (0x0,0,2), end-CHS (0x3ff,255,63), startsector 1, 4294967295 sectors, extended partition table (last)\011/dev/sdd1: data/dev/sdd2: dataTrying to mount it using various partition types, using both /dev/sdd and /dev/sdd2. --ntfs and ntfs-3g:NTFS signature is missing.Failed to mount '/dev/sdd2': Invalid argumentThe device '/dev/sdd2' doesn't seem to have a valid NTFS.Maybe the wrong device is used? Or the whole disk instead of apartition (e.g. /dev/sda, not /dev/sda1)? Or the other way around?exfat:FUSE exfat 1.0.1ERROR: exFAT file system is not found.vfat:mount: wrong fs type, bad option, bad superblock on /dev/sdd2,   missing codepage or helper program, or other error   In some cases useful info is found in syslog - try   dmesg | tail or so.UPDATE 2The partition tables were not recoverable, I had to run a rescue to recover the data. Installing testimage and running photorec worked like a champ, I was able to get back all of the lost data.
Problems mounting GPT partitioned external HDD
partition;gpt
It seems that the drive has been formatted by Windows - which is not surprising, since Windows definitely must have been unable to use the disk which had very likely been formatted by OS X for sole use under OS X. Now the problem is exactly the same, just with the sides swapped.If you want to mount the Windows partition, you can try blindly guess the file system:mount -t FILESYSTEM -o ro /dev/sdd2 /mountpointwhere FILESYSTEM is likely to be (given the size of the partition) one of NTFS, exFAT or (less likely) VFAT. For NTFS one can use either in-kernel ntfs driver (in read-only mode) or the FUSE implementation ntfs-3g. exFAT has (allegedly) working FUSE implementation; VFAT has vfat. In any case consider doing the mount with -o ro or even creating a read-only loop device for the partition and mounting that. The reason for such a cumbersome approach is that some file system drivers may update the file system even if mounted in read-only mode (usually by fiddling with metadata). Which is definitely undesirable.If you want to try to rescue the original (read pre-Windows) data, check the Q&As referenced by Gilles (Recovering accidentally deleted files and How to recover data from a bad SD card?) and search the internet for file system recovery for the file system used by OS X, most probably HFS Plus.As for the general question of initialising a disk: I believe this happens whenever the system doesn't find a partition scheme it understands - this will happen either a MBR partition table or GPT on the disk - or if no partitions it recognizes are of the right type. This can be surprising when one is used to Linux (and I would suppose BSDs as well) which doesn't pay attention to the partition types, instead caring about the actual content only.