usb vs external drive from mac to pc
January 11, 2007 2:23 PM   Subscribe

Why can I use a usb drive interchangeably between my Mac and PC, but not an external drive?

My windows pc is not recognizing an external drive that I use with my Mac, presumably because it is formatted as a Mac extended (journaled) drive, but it makes me curious as to why I can read and transfer files back and forth between them using a usb drive?
posted by horsemuth to Computers & Internet (8 answers total) 1 user marked this as a favorite
I'd guess that your usb drive is FAT, while your external drive isn't.

FAT is fine for small drives, but for larger drives it becomes problematic because it does not support large file sizes. Check the file systems on both drives and let us know?
posted by utsutsu at 2:29 PM on January 11, 2007

Chances are that your USB drive is formatted with a FAT32 filesystem, which both Windows and Mac OS recognizes (Linux and BSD, too). The HFS+ that is used in a Mac extended partition isn't natively recognized by Windows.

If you format the external hard drive as FAT32 (which will destroy all data on it), it will work cross-platform.

(on preview: FAT32 can store files up to 4G in length, which is large enough for most people. The other size limitation is somewhat artificial: Windows XP won't format a FAT32 partition larger than 32GB, however it can mount and/or use ones larger. Conveniently, OSX can format large FAT32 partitions. In the Disk Utility, go through the "Erase" steps, and choose "MS-DOS" as the Volume Format.)
posted by toxic at 2:40 PM on January 11, 2007

If you plan to share the same drive between Mac and Windows machine, use FAT (not FAT32), also known as MS-DOS format.
posted by beta male at 2:40 PM on January 11, 2007

Thanks all. I shall move my itunes library off the drive and reformat.
posted by horsemuth at 2:48 PM on January 11, 2007

Almost all Flash drives, including the cards used in digital cameras and USB "keychain" drives, are formatted with FAT16 (or, rarely, FAT32). This makes them readable pretty much anywhere, since they've been around since 1987 and 1994, respectively.

Hard drives tend to be formatted with more modern filesystems that are more specific to the OSes that the drive is used with. Macs tend to be HFS+, WinXP systems can be NTFS, etc.

To read a HFS+ disk on a PC, you'll want something like MacDisk for Windows. Macs will natively (as of 10.4?) read but not write NTFS.

If you have a drive that you're going to be moving back and forth between a PC and a Mac, you could format it as FAT32, but this introduces some file-size and fragmentation issues that modern filesystems don't have, so it's not something to be done lightly. If you just need to pull some files off occasionally, you're best off leaving the drive formatted with the filesystem that corresponds to the OS you use it with most.
posted by Kadin2048 at 2:49 PM on January 11, 2007

If you plan to share the same drive between Mac and Windows machine, use FAT (not FAT32), also known as MS-DOS format.

FAT (which is actually FAT16) only supports drives up to 2G, but it's widely compatible, which is why most smaller flash drives come preformatted this way.

FAT32 (which is fully supported by OSX as of 10.3, according to Apple and anecdotal experience) is more appropriate for larger drives.

As for compatibility, look at Apple's own product line. All iPod shuffles and nanos ship from the factory formatted as FAT32 (though if you "Optimize for Mac" on your nano, it'll convert to HFS+). If you tell your HD-based iPod to use a Windows format, it also will use FAT32. Shuffles use FAT32 always.
posted by toxic at 3:04 PM on January 11, 2007

Another excellent solution for reading Mac-formatted drives on Windows is MediaFour's MacDrive 6.

(Linux, of course, reads and writes Mac HFS+ volumes natively. I "discovered" this by accident when I rebooted my PC into Ubuntu with the external Mac USB drive still attached, but of course it makes complete sense.)
posted by enrevanche at 4:09 AM on January 12, 2007

I've seen the same thing, but for a completely different reason, namely insufficient power to the external drive when using a laptop. If you're mac is a laptop it may not be providing enough juice. I've seen this with my fiancee's ibook, with a drive that worked fine on a number of PC laptops and desktops...
posted by ThinkNut at 4:12 AM on January 12, 2007

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