there is a tool called chdman.exe it will image the chd to your hdd\
chdman - MAME Compressed Hunks of Data (CHD) manager 0.83 (Jun 5 2004)
usage: chdman -info input.chd
or: chdman -createhd inputhd.raw output.chd [inputoffs [cylinders heads sectors [sectorsize [hunksize]]]]
or: chdman -createblankhd output.chd cylinders heads sectors [sectorsize [hunksize]]
or: chdman -createcd input.toc output.chd
or: chdman -copydata input.chd output.chd
or: chdman -extract input.chd output.raw
or: chdman -verify input.chd
or: chdman -verifyfix input.chd
or: chdman -update input.chd output.chd
or: chdman -chomp inout.chd output.chd maxhunk
or: chdman -merge parent.chd diff.chd output.chd
or: chdman -diff parent.chd compare.chd diff.chd
or: chdman -setchs inout.chd cylinders heads sectors
you can use chdman to re-create a real HDD from a backup CHD image in much the same way:
chdman -extract image.chd \\.\PHYSICALDRIVE3
(remember to replace the '3' with the drive you want to re-create, don't get it wrong or you will completely overwrite the wrong HDD!)
Note that this method also works for drives connected via an external USB box. The above example test drive was a 2 1/2 inch HDD connected via an external USB cable.