Buying my first MBP soon so I want to be prepared. What kind of external hard drive should I use? Are there MBP specific ones? or could I use the one I currently have for my PC? Thanks.
You can use any external HDD available with a USB 2.0 or Firewire 800 port, or even ones with a Thunderbolt port, though they are still rare and expensive.
As it just comes down to the file system you use on that external HDD, there is no need for a Mac HDD, as they just come pre-formatted with the correct file system and are more expensive, but you can just use Disk Utility to format any HDD to the correct file system.
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Overview of the four major file systems (called "Formats" in Mac OS X) used on Windows and Mac OS X, compiled by GGJstudios. You can use Disk Utility to format any HDD to your liking.
FAT32 (File Allocation Table)
Read/Write FAT32 from both native Windows and native Mac OS X.
Maximum file size: 4GB.
Maximum volume size: 2TB
NTFS (Windows NT File System)
Read/Write NTFS from native Windows.
Read only NTFS from native Mac OS X [*]To Read/Write/Format NTFS from Mac OS X: Install NTFS-3G for Mac OS X (free)
Some have reported problems using Tuxera (approx 33USD).
Native NTFS support can be enabled in Snow Leopard, but is not advisable, due to instability.
AirPort Extreme (802.11n) and Time Capsule do not support NTFS
Maximum file size: 16 TB
Maximum volume size: 256TB
HFS+ (Hierarchical File System, a.k.a. Mac OS Extended)
Sim above pretty much nailed everything. I have 2 primary external drives. One (fw800) I keep attached to my Mac for time machine and other external storage. I use hfs+ for that. The other is a shuttle between work and home. I use exfat for win compatibility and have had no issues.