I need to buy and external HDD for my music projects (audio and midi), but I cant use Firewire HDD because I use Edirol Firewire audio interface.
If I want to record multitrack audio and use ext4ernal USB2 HDD as the recording destination, will it be ok and can the USB handle it ?

USB 2 should be fine. you'll run into problems not with the USB bus speed, but with the rotation speed of the hard drive itself. make sure the external drive rotates at 7200 or better.
it's worth noting that firewire chains with audio interfaces and HDs often work just fine. my MOTU 828 mkII has two firewire 400 ports, and i've successfully chained my external recording drive to my laptop through it. it doesn't live that way in the studio, but it's been rock solid on location. worth a try if you've got an external firewire drive handy. if the edirol has only one port, perhaps the drive has two. again, worth a try.
good luck

Are you using a tower are laptop?
Because if its a tower you can buy a pci or pci express firewire portand install it .
if it's a laptop you can daisy chain them from the hard drive if it has two port.
If its not much audio work load usb2 should work fine.
There are also converter cables to convert from one port to another.