Yes, you can unload a dll (see TSystem::Unload).
Valery will probably comment on the problems with relative paths.
Some changes have been made in this area in the coming version 1.03/05.
Concerning your large ntuple file (well a small one 5 Mbytes), I believe
I understand your problem. Your ntuple has 262 columns.
In the conversion program h2root, I am allocating (by default) 32000
bytes
for the buffer of each branch. This means that you need about 8.4Mbytes
of memory to hold all the buffers + a similar amount to perform
the compression. In the new version, I have changed the default
buffer size from 32K to 8K. This should considerably improve the startup
time when you use the ntuple in a Root session. On my machine it takes
now about 2 seconds to read the necessary buffers in memory.
I should probably be more clever in trying to allocate the buffer size
as a function of the number of columns and ntuple size.
Rene Brun