Modelsim Error: (vcom-7) Failed to open design unit file "..." in read mode

Modelsim only does what you ask, so you need to look at what you are asking it to do :)

For example, how are you getting Modelsim to attempt to compile this file? Are you asking Quartus to run an RTL simulation, or are you running a Tcl script?

Chances are you have an invalid filename that you are trying to pass to vcom, eg.,

vcom c:/vhdl/my_vhdl_file.vhd

would work, but

vcom c:\vhdl\my_vhdl_file.vhd

would not, since the backslash is a Tcl escape character, but ...

vcom {c:\vhdl\my_vhdl_file.vhd}

will work, since it passes the path without interpreting the contents of {}

You might also look for spaces in your paths ... they can be used, but you have to use "" or {} to pass the path as a "single string", since that is what Tcl wants to see.

Cheers,

Dave