I have not encountered problems with external HDs.
For printers, you can check http://www.linuxfoundation.org/colla...s/openprinting
to see if your model is supported; if your in the market for a printer, a HP is a safe choice. I have an Epson R300 that worked out of the box if I'm not mistaken and a HP1005 that required the download of a driver from the HP site.
Scanner is another issue that you might want to check. I haven't bothered yet to try to get my Epson 4400 going, but it's not really supported if I remember correctly what I've read. An old HP (3300?) was no issue but died
Without info about the USB modem, there is not much that one can say.
Be aware that office documents might look different (word versus oo-writer). I often encounter that a 1 page document becomes a 2 page document due to different font sizes.
Also excel formulas are not compatible.