A flash drive will work fine. You should be able to put all your music on it at once.
The only real risk of a flash drive is their life span. However, flash drives can have anywhere from 10,000 to 100,000 write cycles before they stop working, so you should be fine.
I store all kinds of sensitive data on my multiple flash drives, and I even run applications off of them.
But I don't worry about them dying because it is far more likely that I will either lose them or buy a better one far before it reaches it does.
This only applies if you get a good flash drive... if you get a really cheap one (uncommon, but I've had some) then it might die of other causes.
However, if you're planning on storing mass amounts of files, it may be better to just get an external hard drive anyway. Or an internal, for that matter. It all depends on personal preference and what you plan to use it for.