Disks is a simple alias for displaying system disk usage. It also demonstrates a bit of 'spaghetti code' with a /goto label jump.

Output

* disks: C:\ | used:  234GB ( 79%) | free: 63.6GB ( 21%) | size:  297GB | (no label) (fixed)
* disks: E:\ | used: 1.81TB (100%) | free: 9.22GB (  0%) | size: 1.82TB | 2TB External (fixed)
* disks: G:\ | used:  917GB ( 99%) | free: 13.7GB (  1%) | size:  931GB | Toshiba Ext HDD (fixed)
* disks: H:\ | used: 4.00GB ( 27%) | free: 10.9GB ( 73%) | size: 14.9GB | READYBOOST (removable)
* disks: ----+---------------------+---------------------+--------------+------------------------
* disks: (4) | used: 2.94TB ( 97%) | free: 97.5GB (  3%) | size: 3.03TB | System Total

mIRC Script (Aliases section)

; Usage: /disks (or) /disks all
disks { ; by Raccoon 2015
  linesep
  var %i = 1, %usedt, %freet, %sizet, %disks, %all, %jump
  if ($1 == all) var %all = $true
  while ($disk(%i).type) {
    var %type = $v1, %path = $disk(%i).path, %unc = $disk(%i).unc
    var %label = $iif($disk(%i).label,$v1,(no label))
    var %free = $disk(%i).free, %size = $disk(%i).size, %used = $calc(%size - %free)
    :disksjump
    var %freeb = $padr($bytes(%free,3).suf,6)
    var %sizeb = $padr($bytes(%size,3).suf,6)
    var %usedb = $padr($bytes(%used,3).suf,6)
    var %freep = $padr($round($calc(%free / %size * 100),0) $+ %,4)
    var %usedp = $padr($round($calc(%used / %size * 100),0) $+ %,4)
    if ($disk(%i)) || (%all) || (%jump) {
      inc %usedt %used | inc %freet %free | inc %sizet %size | inc %disks
      echo -atic info * disks: %path $(|) $&
        used: %usedb ( $+ %usedp $+ ) $(|) $&
        free: %freeb ( $+ %freep $+ ) $(|) $&
        size: %sizeb $(|) $&
        $iif(%jump, System Total, %label ( $+ %type $+ ) %unc)
    }
    inc %i
  }
  if (%disks) && (!%jump) {
    echo -atic info * disks: $+($str(-,4),+,$str(-,21),+,$str(-,21),+,$str(-,14),+,$str(-,24))
    var %free = %freet, %used = %usedt, %size = %sizet, %path = ( $+ %disks $+ )
    var %jump = $true
    goto disksjump
  }
  linesep
}

padr { return $str($iif($3 != $null,$3,$chr(160)),$calc($2 - $len($1))) $+ $1 } ; by Raccoon 2015

(Update) Expanding a little further, I decided to add a column showing the percentage of the relative size of each disk. This required advanced knowledge of the total bytes of every disk prior to main iteration loop, so a second loop is added and %sizet is quickly tallied. Sure, it'd be possible to collect all disk vitals before echoing the output, but that would mean creating arrays to save each parcel of information... maybe not worth it.

* disks: C:\ | used:  231GB ( 78%) | free: 66.4GB ( 22%) | size:  297GB ( 10%) | (no label) (fixed)
* disks: E:\ | used: 1.81TB (100%) | free: 9.22GB (  0%) | size: 1.82TB ( 60%) | 2TB External (fixed)
* disks: G:\ | used:  920GB ( 99%) | free: 10.6GB (  1%) | size:  931GB ( 30%) | Toshiba Ext HDD (fixed)
* disks: H:\ | used: 4.00GB ( 27%) | free: 10.9GB ( 73%) | size: 14.9GB (  0%) | READYBOOST (removable)
* disks: ----+---------------------+---------------------+---------------------+------------------------
* disks: (4) | used: 2.94TB ( 97%) | free: 97.3GB (  3%) | size: 3.03TB (100%) | System Total

; Usage: /disks (or) /disks all
disks { ; by Raccoon 2015
  linesep
  var %i = 1, %sizet
  while ($disk(%i).type) { if ($disk(%i)) inc %sizet $disk(%i).size | inc %i }
  var %i = 1, %usedt, %freet, %disks, %all, %jump
  if ($1 == all) var %all = $true
  while ($disk(%i).type) {
    var %type = $v1, %path = $disk(%i).path, %unc = $disk(%i).unc
    var %label = $iif($disk(%i).label,$v1,(no label))
    var %free = $disk(%i).free, %size = $disk(%i).size, %used = $calc(%size - %free)
    :disksjump
    var %freeb = $padr($bytes(%free,3).suf,6)
    var %usedb = $padr($bytes(%used,3).suf,6)
    var %sizeb = $padr($bytes(%size,3).suf,6)
    var %freep = $padr($round($calc(%free / %size * 100),0) $+ %,4)
    var %usedp = $padr($round($calc(%used / %size * 100),0) $+ %,4)
    var %sizep = $padr($round($calc(%size / %sizet * 100),0) $+ %,4)
    if ($disk(%i)) || (%all) || (%jump) {
      inc %usedt %used | inc %freet %free | inc %disks
      echo -atic info * disks: %path $(|) $&
        used: %usedb ( $+ %usedp $+ ) $(|) $&
        free: %freeb ( $+ %freep $+ ) $(|) $&
        size: %sizeb ( $+ %sizep $+ ) $(|) $&
        $iif(%jump, System Total, %label ( $+ %type $+ ) %unc)
    }
    inc %i
  }
  if (%disks) && (!%jump) {
    echo -atic info * disks: $+($str(-,4),+,$str(-,21),+,$str(-,21),+,$str(-,21),+,$str(-,24))
    var %free = %freet, %used = %usedt, %size = %sizet, %path = ( $+ %disks $+ )
    var %jump = $true
    goto disksjump
  }
  linesep
}

padr { return $str($iif($3 != $null,$3,$chr(160)),$calc($2 - $len($1))) $+ $1 } ; by Raccoon 2015