There is definitely no such thing as unlimited disk space. Disk drives have physical limits. If you read the fine print, they may specify some limit. Most likely they just make it higher than they expect most sites to use (e,g,, 10G). Then if someone actually starts nearing the limit, they'll come up with some other excuse (excessive bandwidth usage, too much CPU utilization, etc.) to force them to upgrade to a higher-priced plan.

I've been chastised on several of those types of hosts for excessive space usage. They will make up all sorts of excuses. They will claim that your files are unauthorized, or your sites are impacting other users, or something similar. Take those unlimited deals with a grain of salt.

well,

There is no appearent reason for wich a hosting reseller should offer unlimited disk usage with no additional charge, of course.
Probably they estabilish a certain space on usage statistics basis among their customers: assuming the 95% of their users use averagely 5Gb disk space or so, they offer you the same space claiming it as "unlimited", relying on the fact that your disk usage will fall within that average, while if you will be one of the remaining 5% that needs more disk space, they assume the risk to loose you, or they propose you a paid upgrade...
But what I'm really wondering for is what is the real disk quota that these "unlimited" packages offers:
what is the limit you came across?
After how many Mb or Gb of disk usage they ring your bell :-D?

If a host tells me I have unlimited disk space I won't host with them because they are telling you a ball face lie, and I want nothing to do with a company such as that.

Find one that gives you limits and you found one to host with.

If you look in the fine print you'll find limits, for example one host I'm aware of limits the file size and the amount of files you can have so it is in effect limited. It's a generous amount but still a limit.

Having said other than what was already mentioned where most people will never get anywhere those limits it's the other limits such as for CPU usage where you'll hit the brick wall. You can have all the disk space and bandwidth in the world but if you're running a dynamic site with something like a forum you're going to butt your head up against that.

Truthfully the only way you could utilize those limits is to have a site with all static html pages serving really large images and video files.