HD question

I recently replaced the hard drive in my MBP and kept the old one and am using it as an external HD.
the problem I'm having is that when I plug in this external HD to a PC it will not show up at all and can't find a way to do so on the PC.
I am wondering if I did something wrong when erasing "formatting" the drive after replacing it.
I hope that makes sense, but if anyone has any ideas I'd really appreciate it.

Thank you,

If you initialized it - sorry, formatted it on the Macintosh, Windows won't read it. You can format it in Windows or Macintosh Disk Utiliay with the Fat32 system and Mac OS X will read it, but you can't format it with HFS+ on Macintosh and have Windows read it.

Because Microsoft refuses to allow Windows to read other formats.

You have to reformat it using Disk Utility using the Master Boot Record scheme, which can be found in the partition tab in Disk Utility in options. Then format the drive for NTFS, and you'll be set.