> 1) Is there a way one can write to a mounted filesystem?
> Just for example, if I wanted to put an executable in /bin...how would
> that be done?

/bin is an empty directory that is then populated by a sequence of binds (you
can find them in /lib/namespace and your own lib/profile). so first you work
out which one of those you'd like to hold the file (ie, global or just your own private bin).
it will typically be one of /$objtype/bin or $home/bin/$objtype for compiled programs
and /rc/bin or $home/bin/rc for rc scripts. copy the new command to one of those.

in general, given a union mount, files are created in the top-most bind or mount that
was bound or mounted using the -c option to allow creation. none of the files
bound to /bin have that option so the resulting directory disallows creation.

having typed all that in i see that pietro gagliardi has already answered but i'll send this anyway.

> 2) To mount a cdrom I know I can start 9660srv...
> but how would I mount another HD? I did not see any other
> filesystem servers.

if a program's primary purpose is to act as a file server, it can be found in section 4 of
the manual, so have a browse through that. some others, including those in ndb(8)
provide a service by serving some files, but that's just the interface so they are documented
wherever the service would naturally belong. (section 8 is admin, which must surely include DNS.)