how to hide partitions on external hard disk

I can see a few ways; the first one strictly answers your question, but I think the second one (maybe in combination with 1) is a better solution:

1) you can mark the partition as hidden. Some operating systems see through this, but Windows will treat it as not existing. If your friends are tech savy / looking for your files, they'll still be able to find them if they try hard enough, but most users won't even know the hidden partition is there.

Partition management software will often do this for you; Paragon's hard disk / partition management software is pretty good, and they often give away old / SE versions on cover disks that will do what you want.

You can also do it with XOSL, which is a free multi-boot manager; if you don't want multi-booting this may be more complex than you need. It's open source and no longer being developed, but is pretty good, and has decent informal support through the Yahoo group; links:
Info and files: http://www.ranish.com/part/xosl.htm
Info and files: http://www2.arnes.si/~fkomar/xosl.org/
Description and comparison: http://en.wikipedia.org/wiki/XOSL
Informal support: http://tech.groups.yahoo.com/group/xosl/

2) you can use encryption software; TrueCrypt is free and popular. It also has advanced features that enable you to even hide the fact that you've hidden anything. This is probably the best solution; and if you don't want people to know you're hiding anything, or even using TrueCrypt, you could combine it with approach 1 above.
http://www.truecrypt.org/

3) depending on the level of hiding you need, you could put your files in a password protected Zip file (or similar). This is generally easier as you may be more familiar with the software (although I have to say TrueCrypt is pretty usable, so if you don't mind learning something new, it's not actually hard), but is less secure e.g. easier to crack, file list can sometimes be seen, may leave files in temp folders etc

If you want more detail, especially XOSL, let me know.

Well, if you want to hide it really good from windows machines, make the "hidden" partition one of the linux types (ext2, ext3, or ext4). You should be able to browse it by mounting using available software. I'd test it first to make sure the mounting/unmounting works well with Win7.