I could be wrong, but I dont think mIRC's $disk identifier is ment to be used the way you are trying to use it.
from the help files:
Returns information about the specified hard disk, where N = 0 for total available drives, and N > 0 to access each drive.
Properties: type, free, label, size, unc, path
$disk(c:) returns $true if drive c: exists, otherwise $false
The unc property returns the path for a network drive.
all it is for is mounted drives, hard disks and CDrom drives, not folder Anything.
the Path you see is relating to a Networked drive not a file path.
maybe what you want is in other identifiers
Returns $true if a file or dir exists and $false if it doesn't.
$exists(c:\mirc\mirc.exe) returns $true or $false.
Returns information about the specified file.
Properties: size, ctime, mtime, atime, shortfn, longfn, attr
$finddir(dir,wildcard,N,depth,@window | command)
Searches the specified directory and its subdirectories for the Nth directory name matching the wildcard specification and returns the full path and directory if it is found.