Hello Freesurfer, 
I am trying to concat my surface pet file using mris_preproc  but I get this error with no output folder or files " mris_preproc --fsgd Pet.fsgd --target fsaverage --hemi lh --is C6_01_001_140813/lh.mgx.ctxgm.C601001140813.fsaverage.sm00.nii.gz --is C6_01_003_140908/lh.mgx.ctxgm.C601003140908.fsaverage.sm00.nii.gz --o all.lh.mgx.ctxgm.fsaverage.sm00.nii.gz
nsubjects = 2
INFO: output detected as nifti, turning on reshaping
tmpdir is ./tmp.mris_preproc.20623
/net/synapse/nt/mozzoude/Analysis
Log file is all.lh.mgx.ctxgm.fsaverage.sm00.mris_preproc.log
Fri Apr  7 10:03:48 UTC 2017
setenv SUBJECTS_DIR /net/synapse/nt/mozzoude/Analysis
cd /net/synapse/nt/mozzoude/Analysis
/opt/freesurfer-6.0.0/freesurfer/bin/mris_preproc --fsgd Pet.fsgd --target fsaverage --hemi lh --is C6_01_001_140813/lh.mgx.ctxgm.C601001140813.fsaverage.sm00.nii.gz --is C6_01_003_140908/lh.mgx.ctxgm.C601003140908.fsaverage.sm00.nii.gz --o all.lh.mgx.ctxgm.fsaverage.sm00.nii.gz

Linux neuron2.sri.utoronto.ca 4.4.0-22-generic #39-Ubuntu SMP Thu May 5 16:53:32 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
$Id: mris_preproc,v 1.76.2.1 2016/08/12 17:55:45 zkaufman Exp $
freesurfer-Linux-centos6_x86_64-stable-pub-v6.0.0-2beb96c
tmpdir is ./tmp.mris_preproc.20623
Src lh sphere.reg
Trg lh sphere.reg




---------------------------------------------------
#@# 1/2 C6_01_001_140813 Fri Apr  7 10:03:48 UTC 2017 --------------
-----------------------
mri_surf2surf --srcsubject C6_01_001_140813 --srchemi lh --srcsurfreg sphere.reg --trgsubject fsaverage --trghemi lh --trgsurfreg sphere.reg --tval ./tmp.mris_preproc.20623/C6_01_001_140813.1.mgh --sval C6_01_001_140813/lh.mgx.ctxgm.C601001140813.fsaverage.sm00.nii.gz --reshape --cortex
ERROR: dimension inconsistency in source data
       Number of surface vertices = 125993
       Number of value vertices = 163842
Source registration surface changed to sphere.reg
Target registration surface changed to sphere.reg

$Id: mri_surf2surf.c,v 1.103 2015/11/05 22:07:33 greve Exp $

setenv SUBJECTS_DIR /net/synapse/nt/mozzoude/Analysis
cd /net/synapse/nt/mozzoude/Analysis
mri_surf2surf --srcsubject C6_01_001_140813 --srchemi lh --srcsurfreg sphere.reg --trgsubject fsaverage --trghemi lh --trgsurfreg sphere.reg --tval ./tmp.mris_preproc.20623/C6_01_001_140813.1.mgh --sval C6_01_001_140813/lh.mgx.ctxgm.C601001140813.fsaverage.sm00.nii.gz --reshape --cortex

sysname  Linux
hostname neuron2.sri.utoronto.ca
machine  x86_64
user     mozzoude
srcsubject = C6_01_001_140813
srcval     = C6_01_001_140813/lh.mgx.ctxgm.C601001140813.fsaverage.sm00.nii.gz
srctype    =
trgsubject = fsaverage
trgval     = ./tmp.mris_preproc.20623/C6_01_001_140813.1.mgh
trgtype    =
srcsurfreg = sphere.reg
trgsurfreg = sphere.reg
srchemi    = lh
trghemi    = lh
frame      = 0
fwhm-in    = 0
fwhm-out   = 0
label-src  = lh.cortex.label
label-trg  = lh.cortex.label
OKToRevFaceOrder  = 1
UseDualHemi = 0
Reading source surface reg /net/synapse/nt/mozzoude/Analysis/C6_01_001_140813/surf/lh.sphere.reg
Loading source data" 

Below are the scripts i ran prior to mris_preproc and the mris_preproc command line: 
1) mri_vol2surf --mov C6_01_001_140813.gtmpvc.output/mgx.ctxgm.nii.gz --reg C6_01_001_140813.gtmpvc.output/aux/bbpet2anat.lta --hemi lh --projfrac 0.5 --o lh.mgx.ctxgm.C601001140813.fsaverage.sm00.nii.gz --cortex --trgsubject fsaverage
2) mri_vol2surf --mov C6_01_003_140908.gtmpvc.output/mgx.ctxgm.nii.gz --reg C6_01_003_140908.gtmpvc.output/aux/bbpet2anat.lta --hemi lh --projfrac 0.5 --o lh.mgx.ctxgm.C601003140908.fsaverage.sm00.nii.gz --cortex --trgsubject fsaverage
3) mris_preproc --fsgd Pet.fsgd --target fsaverage --hemi lh --is C6_01_001_140813/lh.mgx.ctxgm.C601001140813.fsaverage.sm00.nii.gz --is C6_01_003_140908/lh.mgx.ctxgm.C601003140908.fsaverage.sm00.nii.gz --o all.lh.mgx.ctxgm.fsaverage.sm00.nii.gz

Thanks. 
Best,
Paul