Have your startup get you a separate computer. Not mixing work and personal life extends to not doing work development on your personal machine or vice versa.

While they're at it, add in an apartment so that your work computer is not kept at a private residence...

It sounds like this was a summer internship, having to lug a separate computer around because Dropbox doesn't support separate accounts seems like a huge work around. The whole point of Dropbox is that the files are wherever you need them.

That's good personal advice - but it's not really a solution to the problem for everyone.

It's really the only solution if you want to maintain some degree of personal privacy, and don't want to incur all your employer's liability on your personal account (and vice versa).

That's largely going to depend on where you live and the laws of that place. Remember, it's a worldwide market these days!

Dual boot? e.g. Windows/Corporate style Linux for work and Ubuntu/MacOS for home?

How about you just share from the account you created to join the team the folders that are shared with you by other team members with your personal Dropbox account? Now your Dropbox client will sync the shared team folders as well.

On the downside, the size of the shared directories would probably push the size limit of your account, so it's not a perfect solution.